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.
Stud placement is geared toward having support at the ends or joints in sheetrock ,decking and plywood. These sheets are 48" x 96" (4' x 8"). 19 1/4" is sometimes used for roof framing as well.