read this
www.engineersaustralia.org.au/sites/default/files/.../Part%205.pdf
you can get more information in google...
_____________________________________________________________________________________learning is a never ending-process

Actually this could be a deep question, rather than silly. Some guidelines might include placing at least one bearing as close as practical to each radial load; use multiple bearings for long shafts; etc. There are too many possible variations to cover in just a few statements.

There are usually two on a bicycle wheel, and possibly five on a four-cylinder engine. Does that help?

The location of the bearing is depending on your application? What things you will install in the shafts? What are the loads in the shaft? Distribution and kind? What is the length of the shaft? Also it is depend on access ability of greasing or lube oil requirements where it will be located?

Does the shaft has radial loads only or there is high thrust load in axial direction?

Normally and general you will put the ball bearings to sustain radial loads (this must be theoretically counted to have equilibrium system -- reaction locations) and ball thrust bearing at the end of shaft or sometimes at the locations where there will be expected axial movement

thanks a lot to all of you , all your comments were helpful