I would recomend def no more than 4 in there for vegitation but when you get to flowering you might have problems with light and size of grow area to be honest some my advice would be to keep 2 in it and you'll get a very nice crop.
You could go more but i recomend to remove the extra plants before inducing flowering as you might come up against problems in the future..
Let me know what you decide and how you get on dude, best of luck with it...

Take a look at HPS bulbs in a grow room in my first grow dude

Maybe consider a small one. It's worth a thought
My First Grow