I want to get a male Texas map turtle because they are small and I'm not sure how many I can have in a 20gallon tank

I found this: Recommended size for a single adult Graptemys versa would be a 30 gallon aquarium as a minimum (40 or larger preferred). This will provide adequate swimming area, as well as assisting the filtration in maintaining good water quality. For additional Graptemys versa, I would recommend adding 20 gallons of tank space for additional males and 50 gallons of tank space per additional female at a minimum.

Stock Tanks and Rubbermaid containers also work well. The same space recommendations remain the same.

At, Care Sheet - Texas Map

Hope it helps.