how can add a second phone number in my iphone?

Yeah, You'll have to be more specific. I presume you mean can you link your other phones phone numbers, so you can make outbound calls from other phone numbers on one phone that the number wasn't assigned to. you -can- do this, between multiple iphones. If that is what you are asking specifically about.

Please explain.

You cannot have two phone numbers on your iPhone, unless you mean that you want to enter two phone numbers for a contact on the contact app.