Well, to solve this mystery, we're going to have to do some math and a bit of critical thinking.
Grist's +1 ability obviously benefits from a tribal deck. But how much does it benefit from a tribal deck? Each time you hit an insect, you basically double the effectiveness of the ability, in tokens, mill, and loyalty. So if you always hit 1 insect (and then a non-insect), it'd be 2x as powerful as if you never hit an insect. Simple enough.
So how many insects would we need to include in the deck to average getting 2 loyalty/tokens/mill every activation? With, say, 50% insects, we'd have a 50% chance of hitting on the first one, but then if we hit we've got another 50%, and so on, so there's always a potential to high-roll. So how do we find the average number?
After a bit of dredging up the distant past, I've determined that the formula we're looking for follows the geometric distribution - link if you want some light reading. The particular equation we're interested in, luckily, is pretty simple: 1/p, where p is the probability of failure (or 1 - the probability of success) as described as decimal between 0 and 1. Since we've got ~100 cards, each insect in the deck adds .01 to the decimal.
Neat. Now let's put it into action. We're just gonna pretend we have 100 cards instead of 99 cards because the math is a lot easier.
With 0 insects, the chance of failure is 100%, which means our equation gives us 1/1 = 1, which hey presto is exactly how many bugs we would get. Sweet, the math seems to work.
With 100 insects, we would obviously expect to mill the entire deck. Our probability equation gives us 1/0 (since the chance of hitting a non-insect is 0), which is undefined but whatever, we all know that's basically just infinity and anyone who thinks otherwise is a nerd (but, like, a bigger nerd than the guy doing probability to optimize a commander deck). So, whatever, this kinda works and seems like roughly what we'd expect. Moving on to (slightly) more practical numbers.
With 95 insects, failure probability = 5% or 0.05, so the equation gives us 1/0.05 = 20. That is a %$#% of bugs.
With 80 insects, p = 20% or .2, equation gives us 1/0.2 = 5. That's a significant number of bugs.
With 60 insects, p = 40% = 0.4, therefore 1/0.4 = 2.5. That is some bugs.
With 50 insects, or half the deck, p = 0.5, and 1/0.5 = 2. Dos bugs.
40 = 0.6, 1/0.6 = 1.6666 or 1 and two thirds of a bug. Don't think about it literally.
By 20, we're down to 1/0.8 = 1.25 buggy boys. Whoopidee do.
Ok, now for the logic part.
Obviously at the very high number of bugs, the power level is pretty off the chain. Milling 20% of your deck and making an army of bugs will probably set you up for lethal over the next two turns just with the ultimate, and an army even if Grist gets killed. But is that remotely possible?
There are, to the best of my knowledge and searching, no insects that help produce mana to cast grist. The only bug that makes mana is Nantuko Elder, which obviously doesn't do a lot to aid in casting a 3 cmc card. No Llanowar Elves, no Elvish Spirit Guide, no Kazandu Mammoth // Kazandu Valley, not even a Blood Pet. Since none of our mana sources are insects, that means to have 95% insects we'd only have 5 mana sources in the deck. To make matters worse, there's hardly anything that can produce more than 1 of the mana in grist's cost - only one that comes to mind is Lion's Eye Diamond (which, tbf, we might as well use since we're sure not casting anything in our hand anyway). What are the odds of drawing 3 mana sources out of 5 in the deck? Yeah, I'm not doing math on that, it's obviously unbelievably terrible and will never work. Moving on.
At 80 bugs, we might actually be able to cast Grist on turn 3, or even turn 2, semi-consistently with mulligans. But, we're down a lot on the number of tokens, to an average of 5. Is that a game-winning number? Eh...probably not. You can ult right away, but it'll only deal 4-ish damage (4 bugs and a mana source, which is probably a land). Even if you get lucky and hit 10, it's probably not game-winning, and if anyone kills Grist you're basically f'ed in the a since you'd need to draw 1/4 of your deck to cast him again. Plus you're definitely not guaranteed to cast him still - if you keep a 2-lander, you've got solid odds of bricking a nonland for a good long while, and a 3-lander is super unlikely - even considering you can basically mull down to 3 relatively happily. Anyway this might win very rarely but it's basically a terrible gimmick deck at best.
At 60 bugs, we have a real-ish deck. We can cast grist quite reliably and maybe even run some cards that aren't bugs. What have we gotten for it? Well, we get 2.5 bugs per activation. That's strong for a 3-drop walker, don't get me wrong, but at this point I'm going to have to do an aside about the insects that exist within the format.
They're absolutely terrible.
Here's the search I used - link. It started off ok with some ok-ish cards. Mazirek even seems pretty synergistic, although wirewood symbiote is obviously a miss here. Giant adephage is...pretty bad...there's some terrible cards only being used in the morph decks...some infect dudes...by #25 we've hit Kazandu Nectarpot and my eyes are officially glazing over. These are so, so terrible. Any deck with 60 bugs is basically throwing away 40 slots of absolute jank, and for what? For 1.5 extra bugs a turn that you can't even exploit because your deck has no payoffs. Because as far as I can see, there aren't any tribal insect payoffs that are insects. The closest we've got is Bloodline Pretender, which isn't exactly bowling me over.
With 40 bugs, or 20 bugs, there's plenty of room for payoffs, but there's not much point. We're getting a fraction of an additional bug every turn. Who gives a crap. Insect tribal is stupid and terrible.
But wait, one last ray of hope - Maskwood Nexus! "Maskwood nexus will save us!", you say. No. Wrong. Even if your whole deck is creatures, you're still only getting 2.5 insects per activation, after you've searched up a singleton artifact that costs 4. Even with dtutor that's 6 mana to get an extra 1.5 tokens per turn. You could just play, idk, some other better token generator, make more tokens, and skip the rigamarole.
Okay, fine, you say. But insects was never supposed to good, it's just a fun gimmick commander! Lean into it!
No. Also, shut up. The point of all this is that Grist tribal is a failed state and we have to move on from the ashes.
Fun fact - grist is the only straight BG commander that mills cards from your library. Many of them pay you off for self-mill, but Grist is the only one who can actually get us going. Plus he's a payoff with his ult, plus he makes tokens, plus he's a decent control piece with his -2. These are all totally reasonable things to build around that don't require any
(or at least, like, not very many.)
Great, glad we're on the same page.
Self-mill is obviously a versatile and powerful strategy within BG, but Grist being pretty open-ended means we can lean into it without having any particular bent to satisfy our commander's requirements, since he doesn't care what he mills (RIGHT??!?!). There are a lot of cards that provide value from the grave, from flashback to incarnations, but the most common in my estimation is the category of creatures which self-recur from the grave. This fits nicely with our token production to give us a large army of expendable creatures, which we can happily throw into a wood chipper, a.k.a. sac outlets and death triggers galore. And since we've got so many graveyard-happy cards, we'll want to lean hard into dredge cards to fill the yard quickly and frequently.
So here's where I'm at so far - the deck is mostly creatures, which plays nicely into cards like Genesis for recurring stuff from the grave, but there are some noncreature cards *cough* Survival of the Fittest *cough* that are just way too good not to include. I was initially worried that I wouldn't have any room for answers, but I was happy to see there's a decent number, in addition to the commander's -2.
I'm still a fair few cards over, so I could use help on a few more cuts. I think it's 73 cards atm (haven't done lands yet).