Under the Rules of Golf, a golfer is allowed to change their golf ball on every hole -- if they're so inclined. Of course, that would mean a golfer or their caddie would have to carry 18 golf balls in their golf bag. Plenty of golfers do that anyway because they expect to lose that may golf balls in a given round, but that's still a lot of golf balls to carry.
Professional golfers don't carry 18 golf balls in their bag for a tournament round. They're way too talented to need to carry that much extra weight in their bag.
However, professional golfers also don't have just one golf ball in their bag for a tournament round. So, it's somewhere in between one and a full 18 -- maybe a box, or a dozen, is a common number. That varies by the golfer.
Professional golfers also have different views on how often they should change golf balls during a round.
For example, Ernie Els changes golf balls every three holes. That means each golf ball is used for three holes before being retired.
Some golfers do the opposite equation, changing golf balls every six holes and using three in a given round -- provided none are lost.
Other professionals really don't have a particular preference or superstition. Rather, they'll change golf balls if they inspect the ball and see something wrong with it that might impact its performance. Some of those imperfections might be a cut or a scratch on the ball's cover. It could be fraying of the cover. Maybe it looks like it has lost its shape for some reason. At that point, the golfer swaps out the ball for a new one.
Damaged golf balls can impact a player's performance significantly because they may spin differently than expected and not travel the same distance a golfer would anticipate. It's key for a golfer to know their ball will do exactly what they need it to do on each and every shot.