How many 400 watts metal halide do I need for a 7500 square foot gym nasium

... so the number (4) is the universal answer to all the questions in the universe...

If you would like a better answer, tell us more about the geometry of the space, for example how high is the ceiling? The length x width measurements of the space? If you are in the USA you will be limited by the energy code... is this for school or your job?

Refer publications from illumination engineers society for illumination level(lux)required for a gymnasium and calculate. If you make a mistake you may get fired so go to an illumination engineer & get trained.

or you could assume that the energy code (if you are in USA) would force you to remain somewhat conservative in the amount of light you'll be allowed to install...
ASRAE90.1 - Gym Playing area = 1.4 va/sq ft * 7500 sq ft = 10,500 va
10,500 va / 500 watts per fixture (assume boyfriend = 1.2) = 21 fixtures...
Now the disclaimer - you did not give us enough information which makes anything with a variable impossible... if you follow up on your question I'm sure we could both learn something...

Won't the height of the gym,types of surface of walls,floors etc play any role in the evaluation?

Absolutely! An unwritten part of the disclaimer. (21 fixtures is just the maximum
Allowed by the code regardless of other necessary considerations.)

It's a shame so many never follow up on their questions...