In general no single supplier should provide more than 35% of any source. Anyone beholden to a single supplier is opening themselves up to be ripped off at best.

Problems of course come when suppliers merge. I have contracts with 8 different supplier for my business, one goes bust, or jacks up prices, and I simply increase supplies of the other 7.

Then those 8 companies start merging, and before long you're left with a single supplier, or at best a cartel.