you can have 10 shrimp to the gallon.. but they multiply so quickly i wouldn't recommend it.
what i would do
have white play sand as substrate
have spider aquarium wood (small) that is angled 45 degrees from one of the comers (adds depth)
make it a planted tank, have dwarf hair grass for front ground.. and it spreads nice, small breeds of sword like plants in the middle ground near the wood or beside and have tall grass for the background plant. but if you want to do planted you'll need all the requirements to make sure they thrive.
and when you have all that done i'd put in 5 red cherry shrimp and another 5 bee shrimp or yellow shrimp.