All depends on your style of growing and how long you want to veg for. I would run around 220 20 gallon pots in 3 rows of 2. Just try it and if u want a shorter veg and smaller plant add more. If u want a bigger plant, take some out.
But with 20ft wide houses, i always do 3 rows so there is 2 walkways.
You should expect to pull about 100lb a run.