3 plants across in either 2 or 3 gallon pots
Or
2 plants in 5 gallon pots
I just got that exact size tent and plan to run 3 plants across in 3 gallon pots, using a smaller strain
Depends mainly on strain and pot size, unless your going hydro (Dwc) than 2 would fill that bitch up easy if treated well
It's kind of tough due to the depth of 20", that extra 4" would allow you to put 2 plants deep