Neon tetra's are a schooling fish and like to swim in packs so as to not feel threatened by predators. The tank is a species tank so there would be little to no reason to feel threatened other than bullying. If a fish is being picked on or bullied, then I would add 4-5 more tetras just so they feel safer. I used to have a similar sized tank with 10 neon tetras and they were fine. But unless some fish are being picked on, I would see no reason to add any more. That is only for the tetra tank though as I have no experience with guppies.