Denison Barbs should be kept in groups of 6 and up.
Nice use of the 1 inch per gallon rule.
However, the 1 inch per gallon rule is a good thing for beginners, this way they will not over stock, but, this rule is merely a guide line.
What you want to take into consideration is how much ammonia your fish produce, and you filters turn over rate and ability to process ammonia.
Yes, there is a limit on how many fish can be in a tank.
Also, depending on your filtration, the amount of cover in your tank, and your tank mates, that rule can easily be extended to 2 inch per every 1 US gallon.
-FHM