bittrex exchange used to have a tab called "distribution" or something like that which showed how much of an altcoin anyone holds in their bittrex wallet. they sadly removed it
what you want usually is found on altcoin block explorers. it can be only for one coin and they have the "rich list"
the only thing that I have in my bookmarks is this:
https://chainz.cryptoid.info/xmg/#!wallets
if the link didn't work because of the # tag go to the "largest wallet" tab and there you can see the list you want. this explorer has multiple coins too.
