how many plants should I put under a 250w hps light?
It depends on how big you grow the individual plants and/or how you train them. 250w HPS is good for around a 5 square foot canopy for flowering. Maybe a 2 foot by 3 foot area max.
Light print of a 250 watt is generally 2’x3’ like mentioned above. Keep in mindGiving the plants a bit of space is good for them. I would not grow over 2 plants in that space, unless you have a really good training regimen planned.