any recommendations if more than 1 hawkfish in a 20 gallon tank?

Answer 1

My recommendation is not to do it.

There isn't enough space for more than one of the family to keep them from fighting. Anytime you mix members of the same marine family, or that have similar body shape and coloration, they will see the other as a "rival" to be driven out. The natural territory size for these fish is larger than the size of most commercially available tanks, so one per tank is the limit.

Answer 2

The minimum tank size for a hawkfish is 30 gallons. However, I disagree with the don't keep more than one. They are best in pairs (male and female) or in harems with one male and multiple females.