Where does it end?

[DirkGently]

Good luck though, organizing a collection of 10s of thousands of cards is so damn boring XD.

Ain't that the truth. I'm at around 8k and that's too tedious for me already. I like to keep all options available though, so I'd bet the minimum is probably at least 500 per colour even if you cut off fringe stuff.

[capitacommunist]

I think I'm at 200-250 per color currently, but even then there are quite a lot of cards that I have never played/considered in deckbuilding.

[LightningHelix]

For example, this is what I cut down to in White for my collection (I changed it to 180 cards.) Pretty much all the cards I'd play plus a few extra.

https://scryfall.com/@WhiskeyBepis/deck ... a0713ce5b6

EDIT: Also, no Legendary creatures here, that's for a different Section.