how many plants can i fit in ?

This is my first grow i have just finished my room i have done as best i could with the money available.my room chosen is 900 mm deep 750mm wide and 1800 tall. In there i have a 5inch inline fan wiich provides exelent circulation and a 400 w hps bulb in a cooltube. It is decked out in what looks like is basically heavy duty tinfoil from hydro store. And a rhino carbon filter. How many plants would you all suggest putting in there for best yield. And also is the hps bulb good for vegging as the gentleman in the shop told me it would be, but i would like to hear your opinions. Thanks alot for answers people greatly appreciated. As i am a newbie.

how many square feet do you have? 400w hps you should be good for awhole lot of fun

well, as many as you want. you can do it very nicely either with just one or two really big plants in really big pots, or you can use dozens and even more of small pots and sog them. read about the different grow methods (especially sog and scrog, and most especially vertical scrogs!) that can maximize your yield in different setups. Remember, it's not about how many plants but mostly about how much total space you have for the roots, and how well you make use of your light. these are the most important factors you should decide about now, so PLAN AHEAD. I didn't, and no doubt it is costing me serious yield.

Also, HPS is great for vegging. I have had excellent results and 3' plants in 6 weeks. Another important thing is that you don't need or want your plant to grow tall. You need to maximize the concentration of light on a layer of about 1 ft of vertical space where your buds will be (if your doing a horizontal, i.e, regular grow). more than that is a waste of space, time, and lots of other shit.

good luck to you.

and remember the single most important noob advice: overwatering does not mean giving too much water at once, but rather it means not letting your medium dry out between waterings

I asked the same question as you when I first started, I was thinking more plants more buds bigger more $$ but it's not true I also have a 400w lamp but my grow space is about 2 meters sq I can get the same off 3 big plants if I was to do 8 small ones I like to veg for 5 weeks then flower for 8, you will give the plant more room to do it's stuff. Over crowded grow room a big no no, good luck with ya grow.