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I think 12 works seeing that the males have a much smaller bioload than their wives. All male Endler displays are as good as it gets with livebearers. I always have folks wanting all males. I call them male color packs and typically put between 2-4 color strains in each. The only other advice I'd give you is feed high protein flakes and make sure to feed several small meals daily. Don't count on the shrimp to eat too much excess food. I'd get some Ramshorn or Pond snail to help if necessay.
I would suggest starting with six. 3-3 each sex. They will reproduce and regulate the population. For plants use dwarf hair grass or something similar for fry and shrimp to live in. As long as you keep the water clean you could have up to 20 at a time.