If I had a 20 gallon tank...I would probably only want two full grown garters in it...but that is just my opinion. You can put two of most types of garters together...other members can tell you if there are certain subspecies that should not be housed together. You would want to have two of the same size...for the most part...you risk problems if they are different sizes. You also do not want a male and female unless you plan to breed...and unless you have experience with breeding, you shouldn't really breed as it is quite an undertaking. I think your best bet would probably be two females...but, again, that is my opinion.