If you stick to only females, you could probably still add 2-3 more but I would not reccomend it unless you realy, realy want them and are prepaired to remove some if necessary. However, don't add any males. It would be too crowded for males. A single male with the 4 females you have now is possible but it would have to be your very last gourami addition as most males won't tolerate other males of pretty much any similar species and they don't like being crowded. Also, there is no minnimum number for keeping three-spot gouramies - they do fine as solitary individuals and don't appreciate being crowded as they like to establish their own territories. I wouldn't add any other species of gourami either. There are exceptions - for example a moonlight or two would be ok if you stick to the 4 female golds you have and can ensure that all the fish will have places to hide etc (moonlights get to 7" BTW but are peaceful). Avoid adding any more three-spots (that is blue, gold, opaline, cosby, platinum, lavender etc - color morphs of trichogaster trichopterus) or any smaller gouramies such as those of the colisa genus as they will likely clash or be bullied respectively. In a 55 gallon, I would say you can fit 6-7 gouramies in max. but you are better off with only 5 golds/three-spots at most.