Cleaner shrimp such as peppermint shrimp are all hermaphrodites, both male and female. So, all you need are 2 shrimp and they are likely to spawn. Both shrimp usually carry fertile eggs at the same time, and seem to spawn the same time as well. The shrimp fry are for all practical purposes impossible to raise.
edit: just thought of this, why is this in the photography forum?