how many sides should one polygon has??
 

from what i read ..some say 3-4 will be good ...but some body said it doesn't really matter

Rule of thumb is that you should try and have as many 4-sided as possible... I'm not really sure when non-4-sided is a problem, but they can give freaky results if you're smoothing them, and MR won't render sub-d's with 3-sided...

You should try to keep them to quads or tris. Anything higher may give unpredictable results in modeling (non-planar and manifold, etc) and rendering. When some one says it doesn't matter, they're probably referring to the fact that all renders are tessellated (triangulated).

pentagons are nice to sub divide, and they take up less geometry per area smoothed than quads. unfortunately they have to litterally be a pentagon shape whicih makes them harder to model with in maya.