There will be 13 studs in a interior wall and 15 studs in an exterior wall. The reason for the to extra studs in the exterior wall is for drywall backing in each corner.
If you have a studfinder, those are really helpful. If not, sometimes you can find them by knocking along the wall. The spot where the stud is will sound less hollow.
Rap on the wall with your knuckle. Step 2 Move sideways along the wall, still rapping, and listen for a solid sound. Remember that studs are usually placed 16 or 24 inches apart - but you can't count...