Hi :)
All the micro fiber inserts I have only required one washing to prep and they were ready to go.
As far as cotton goes, yes, it needs several washing to prep them so they are absorbent (if you get brand new cotton prefolds they will be really flat, and after several washes they get soft and fluffy and quilted). It depends too if you get unbleached or bleached cotton, as bleached requires fewer prep washes. I'd say 4-6 prep washes for bleached and a few more for unbleached cotton. You can always test them after a few washings and dryings by putting a little water on them and seeing how it absorbs. In my experience, it's taken me less prep washes than what most manufactures recommend, but again just test your diapers for absorbency.
For hemp, I wash them 4-6 times in hot water, but you can test these too. Oh and use hot water for prepping and dry in the dryer at least a few times between all your prep washes. This is what has worked for me, I hope that helps :)

Maybe try the manufacturer's website, they usually have care instructions.

If it's a rental trial, typically you just wash everything once and the company you are renting from provides the care instructions they would like you to follow. That's how ours worked.