I think I was told min as 24 so you have enough for bub to wear whilst you wash and dry one days worth. It all depends on how often you want to wash and how much you can afford buying at a time.
I built mine up so that I could go a couple days without washing though he was older by then and not using so many in a day.
Any of the stockists or businesses can answer all your questions if you email them.
As for bulkiness, that is why I liked using the itti bitti and MiaMoo ones. He was in large when we started so I loved the osfm not being bulky on him. I'm interested how they will go on this baby when I have him/her