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Its depents on area of steel , contrete and clear covers, type of cross section, based on loads we can calculated.
It depends upon the structural member which you are intended to design .
There are some codified provisions in IS-456 (2000) which will help you out in determining the no. of main steel bars.
1. Rectangular columns-4 sides
2. Circular columns-6 sides
3. Octagonal columns-8 sides