Barbonius get very large and are absolute vacuums, they eat everything and always want more, so to your question of if this is too many fish, it all depends on what your tank can handle. If you have a skimmer 12 is fine, I had almost 30 fish in my 135 g and felt comfortable with both the swimming space available and the bioload. It’s important that you take into consideration what levels of the aquarium the fish are likely to reside, as if you have all benthic fish then the tank can be quickly overstocked, and same with if you have all pelagic fish that like to swim towards the top of the tank. Long story short I think your at a good number