Under the Rules of Golf, a golfer is only allowed to carry 14 golf clubs. They can be any 14 golf clubs, but they can only carry 14 at a time. But what about golf balls? Do the Rules of Golf dictate how many golf balls a golfer can have in their bag?
As it turns out, no. Under the Rules of Golf, a golfer can carry as many golf balls as they want in their bag. Really, they can carry as many golf balls as they're willing to carry around themselves or their caddie is willing to lug around for them.
Most PGA Tour golfers carry in the area of nine golf balls in their bag per round. Some golfers like to change balls after a specific number of holes -- one, three, six, whatever -- and so they have enough golf balls for that choice and a few extra in the event they lose balls in the course of play to a water hazard, out of bounds, a lost ball or damage to the ball during play.
In PGA Tour, most professional and high-level amateur tournaments, golfers are bound by the One Ball Rule, meaning they can only play a single golf ball -- same manufacturer and model -- throughout a stipulated round. So professional golfers can carry as many of the same golf ball in their bag as they'd like.
The key in golf is to not run out of golf balls. It's hard to play golf without golf balls. However, if a golfer runs out of golf balls during a tournament, they are allowed to borrow golf balls from another player, run to the pro shop and buy some more, or they can procure them through some other means, so long as the time to get more balls doesn't cause undue delay in the round. Golfers are only prohibited from borrowing each other's clubs. Borrowing golf balls is OK.
Of course, if a tournament is being played under the One Ball Rule, the balls a golfer borrows from another must be the same manufacturer and model the golfer has been playing throughout the round.