Changing number of rings to answer on iPhone

There isn't a setting to change the number of rings.

The period of ringing before the iPhone has voicemail answer is not based on a number of rings, it is based on time. The carrier sets a time from the setup of the call before it switches to voicemail, generally 30 seconds. You can contact your carrier to see if they can adjust it any, some can and many cannot, but if they can, it is not more than a second or two.