Welcome to the forum family! I have a 20 long tank and keep 5 females. 3 of my females are a smaller garter though. I have 4 T. ordinoides (Northwestern garter) 3 girls and 1 boy, 2 female T. s. pickeringii (Puget Sound garter). We have a lot of climbing stuff so it adds more space. My little boy NW is only 4mo olds so he gets a 10gal. all to himself for now.