I’m assuming you either meant the saltwater clownfish and either didn’t see it or didn’t care that there is a saltwater section or you meant a clown loach. For the saltwater clown fish 20 gallons I think and for the clown loach you could keep one in a 55 for years because they tend to grow so slowly.
Assuming saltwater clownfish, there is some debate on minimum tank size, general consensus is 10g for one, 15 or 20 for 2. Then, there are some people that keep them in 5 gallons. But I would go for a 10 at least to be safe.
Yes you can put more fish in a twenty-nine gallon tank. We had a Flame Angel and several smaller fish in a 29 for years. The angel was boss and kept order in the tank. Beware of damsels though. They are tempting because of their size and price but they can be mean.