Mantis "shrimps" (they're not really shrimps, different Order) -- one per tank.
Other shrimps -- depends on the species. Cleaner shrimp -- usually unlimited. Coral Banded Shrimp ("boxing shrimp") -- one unless you've got a mated pair, then two. Pistol shrimp -- usually one, sometimes can get away with more.
Note that stomatopods ("mantis shrimp") usually require a specimen tank. Usually people put soft corals and maybe a "throwaway fish" or two (like damsels).
Dan