Try this:
See how many clothes they go through between laundries. You might try to string things out EXTRA long between kids-clothes-containing loads just to give yourself some margin of error. OK, however much clothes are in the laundry when you break down and decide you're gonna do it, that is sort of their minimal clothing needs. Count the shirts, pants, underwear, socks.
Then add a few extra to your total for 'what if they played in a lot of mudpuddles this week', and maybe add a couple other carefully-selected outfits for specific occasions that you know will come up occasionally (grandparents, restaurants, really messy hikes, etc). Something that wears out frequently like pants you would want to add in a few extra pairs to account for replacment.
And there ya go. That is approximately how much clothes they need. Their needs are likely to be quite similar in the next size or two up, too.
I think it'll differ a LOT among people because everyone has their own lifestyle, attitude towards how worn something has to be before it gets washed, and how often you do laundry. For instance each of my kids (almost 2, and almost 5) has exactly one pair of shoes (plus snow and rain boots), whereas I know kids who have, I am not making this up, twelve pairs of shoes and the mother claims they are all worn regularly.
Good luck, have fun,
Pat