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Hi I am a new user. can any body suggest me that how many number of layers I have to consider.

It depends on your task. Check how your results change with the number of layers, thickness of the gap between free surfaces and area of your surface. You can start from 6 layers and then increase the number. The gap should be about 3-4 lattice constants, otherwise your free surfaces will interact with each other due to periodical boundary conditions.

To add to what pavel said, the gap size should be about 10-12 angstrom (bigger is better but more expensive). If you want to study surface adsorption, make sure the space between slabs is big enough to accommodate your adsorbate PLUS 10-12 angstrom of empty space. Six layer slabs are good for metals (though I find that heats of adsorption are often converged after even 4 layers). For semiconductors, you may be okay with fewer layers.

Thank you very much for your kind informations