Set up

Ive grown before but used CFL's. but meh. just grabbed a blast, 400-watt HPS 400 watt MH, grow tent, etc etc. How many plants should I be able to get under the reflector using 400 watt bulbs ?

1 or 2 i would say.

i can cram ten under my 400W HPS (i also have a small T5 for additional lighting in a "dark" corner). congratulations on the HPS - coming from CFLs you're going to love the difference. i started out with a T5 for veg and an HPS for flower and initially was always grateful i didn't have to mess with the CFLs seeing how much work they are! growing cannabis is a lot of daily work anyway--much less adding to the daily chores having to move your light, etc. your buds are going to probably be a lot denser too. bonus. so back to your original question, as many as you can fit in your flowering tent. in my flowering area, i can cram up to ten girls in there in various pot sizes.

ETA: prefer to run four girls in 3-gallon containers so two to four is probably a good start for you

I run 5gal bubble buckets, used to be 5-7gal smart pots... Go with 4 plants under the HPS, use the CFLs for seeds/small veg plants so u can stagger the flowering plants...