If I apply 12 colleges using common app how many essay I have to write???
It depends on which colleges you applied to.
At least one. Some schools require additional supplemental essays.
You will most likely have to write supplemental essays for many schools. However, you can tweak some and use them for multiple schools if the essay is a good response to a prompt.
For example, one additional essay prompt required by several schools S applied asked for one additional point of interest (about him) that wasn’t covered on the common app. He wrote a humorous and engaging that could be used more than once. Just be careful to edit any references made about the school. Make sure you get the right one in there.
My daughter applied to 8 schools and wrote 19 essays. One of her friends only had to write the common app essay and an honors college essay. Totally depends on where you are applying.