What's the smallest rear cog and biggest rear cog you can get?

Phil do a 12t but anything smaller your chain would be bouncing off the lockring i think

I've got a Suntour 12t, and it only works with some lockrings, can't remember which ones now. It would also depend on which chain you use, as they don't all have the same height side plates. E.g. DID NJS chain plates are 9.5mm, but SRAM PC971 (3/32") are only 8.4mm