Get a few if you want to end up with one or two if you are lucky. That is unless they are tank raised. Bangai's these days are not very hardy and the majority of wild caught ones die within weeks. I'm not sure what's changed in recent years and whether it's where they are being collected or the collection methods but years back, they were one of the easier fish to acclimate to tank life. Now, not so much. This has not been my experience, I have found that about 10years ago wild Bangaii cardinals were very delicate and difficult to keep,and in the last 6 years or so have been extremely hardy and the only ones that die is after some have paired up and killed the others. But it could also be supplier related.
How large is your tank? If it's 60G+ I'd start with 4 or 6 if they are store bought. You will likely end up with a pair or possibly 3. It took 6 Bangai's for me to end up with 3 in my main display and 2 resulted in one Bangai in my display fuge. My system is very healthy too. Pajama Cardinals on the other hand are about 100% success rate for me. My 3 Bangai's in my main display and 6 PJ's constantly breed and the males will carry the eggs in their mouth. Once they acclimate to tank life they will breed constantly if you have a few. I do believe they change sex so one will be the dominant one and the other will become the females.Nope, Bangaii cardinals are born male or female and do not change sex. Males like a harem so typically you will end up with one male and the rest of the survivors will be the females.This usually doesn't work out either as Bangaii's of the same sex don't tolerate each other, males kill off other males and females kill off other females