1 per gallon is a good starting point for ghost shrimp. You could certainly keep more - I think 5 is a good number. There is a certain number of shrimp that your tank can sustain without changing a thing (primarily how much you feed). For example, if you got say 10 shrimp, you might find that after a couple months you only have 5 because there's not enough food available to sustain more than that. If you were to feed the shrimp (over feed a little), then more will survive. Same principle as with the pest snails.
125 - BGK, chanchito cichlid, pictus cats, silver dollars, palmas bichir
125 - cichlids (severums, bolivian rams, chocolate), rainbows ( turquoise, red), loaches (angelicus, zebra, kuhli and horseface), plecos (BN, RL and clown), denison barbs, tiretrack eel, pearl gouramis, betta
90 - Congo tetras, african knife, upside down cats, spotted ctenopoma, kribensis, delhezzi bichir
2.5 - betta