Look into SOG. Consider that the HPS will only penetrate 18". Its not productive to finish plants tall so you want to plant dense and 2 week veg max, for rooting really, not to get them bigger - you plant dense. How many depends on the genetics, more if they are vertical, sparse growers or less if they are bushy and side branch extensively. Anywhere between 9 and 16 depending on those factors.
thank you for the reply i was thinking around a 2 week veg as well, and i have the spacing setup for 9 spaces 20' square in my 5x5 so i was thinking 9 as well