How many plants is a question that gets asked all the time - it's the same as asking "How long is a piece of string?".
How many plants you can grow in any given space is purely a function of how you're growing (Lollypoping, Sea of Green, SCROG, etc.). You could, easily, have one plant covering 3 sq. ft. - or have nine plants (or more!) growing in that same 3 sq. ft. space. Lights can only be expected to cover a certain amount of area (sq. ft.), how many plants anyone actually grows in that area is up to them (according to the size of plants).
Now, having said all that, the size of light you want to buy is totally dependent on how much space you want to dedicate to growing. You can figure on about 50 watts per sq. ft. of grow area.