The most betta splendens that you can put into a tank permanently is one. That tank would hold a lot more, but betta splendens do not play well together. It is the main reason that people keep them in small tanks. I keep a nice 4 year old male veil tail betta in a 45 gallon tank with my endler breeding colony and he fits right in.
Black neons would make a nice starting stock for a tank that size as long as you clone your present filter. Another nice colorful but small fish is rasboras, but they would also need to be in significant numbers, just like the neons. A betta may or may not be OK with the black neons. I have kept the two species together in he past but many people are concerned about the neons being too active to easily keep a betta happy with them.
I have been exploring the wild type bettas lately since we got the new section on non-splendens bettas. I really like the look of some of those and most of them can be kept in groups, which means you don't devote a whole tank to a single fish. It is just food for thought. I will be checking my LFS for Betta falx today when I go there. They are a small mouth brooder that I find beautiful and their description makes me feel they would be a challenge for me that I have not had before. Breeding Betta splendens is very easy and I did it when I was about 10 years old. The only problem I see with splendens is the need to maintain so many separate bottles of water in good condition once the fry start to be able to be sexed.
