The general consensus on mandarins is that they need a mature tank that has at least 50lbs of LR but that might be considered low. It's rare but some have mandarins that will accept other foods but their main diet is copepods which live and reproduce in/on LR. Crushed coral will be a headache it's just a matter of time.
I don't think that there is a set number or that anyone can really help to determine exactly what your clean up crew numbers should be. Every system is different. Currently I have a 29g tank and I've re-upped my cleaners several times over the last year. I don't have much algae and my tank can not sustain a large number of cleaners but at the same time my blue legs like to eat some of my snails. I started with 10 astraea, 1 mex turbo, 10 nassarius, and 10 mixed hermits about a year ago. Most of the snails have died and have been replaced but currently I have 4 astraea, a few nassarius, a few cerith, and 7 hermits.
FYI- Astraea's can not right themselves when they fall on their "back."