If you but the bum G brand ones, then I think they are microfibre and dry really quickly, so you could just get away with a days worth if you wanted to wash every night. So 6 -8.
But you can stuff a pocket nappy with anything, hell, you could use small terry hand towels or tea towels if you really wanted.
Maybe you should get some bamboo prefold/trifolds and use those, you will find you can re use them in most pocket nappys, you will just need to figure out what size rise you will be using the nappy on (ie what length from belly button to top of bum, between the legs KWIM?, not around the waist) and make sure that the prefolds you buy to not exceed that length, so they stuff in ok.
If you buy the proper insert, they have snaps so you can size them down, but you will just need to know what works for you price wise. There is a FB page called buy and sell your MCN, have a loot at what you can get 2nd hand to try, there are usually lots of inserts/prefolds/trifolds/boosters to buy.
GL
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