Can I use rebar 4 length-12mm dia. for square columns (12ft).?

Question

The columns are to be constructed at my car porch area which is about 32ft x20ft size.

Answer 1

If you are designing this yourself, and just guessing at the reinforcement, the answer is no.

If a qualified engineer has designed it and you are having doubts about it (at first glance it seems too little to me) you would have to give more information for anyone to check. The size of the column (you seem to have given height, but the dimensions perpendicular to the height will be needed too) and the load applied to it would be needed to even start to check the design.