The Masters is the only one of the four men's major championships in golf that doesn't have alternates. If a player withdraws or is unable to compete, they're not replaced by another player. Instead the field is simply reduced by one and, if the withdrawal happens before the tournament, the tee times may be adjusted accordingly.
So, why doesn't the Masters have alternates, while the U.S. Open, British Open Championship and PGA Championship all do -- and typically use several of them during the championship to complete their field?
The Masters has no alternates because it is an invitational tournament, not an open tournament. That means Augusta National sets the parameters by which they invite players to compete -- one, for marketing, and two, so pros know how to get an invitation -- without setting a maximum field size. In modern times, the Masters field would be unlikely to be under 90 players, but it's remained below 100 players. But, if for some reason the Masters wanted to only invite 50 players, they could do that.
There are invitational events on the PGA Tour, but they are handled differently than the Masters, which was originally called the Augusta National Invitation Tournament. PGA Tour invitational fields have a certain size posted before the event field is completed, and the Tour publishes the criteria by which a player can earn an invitation. These fields are filled to capacity with PGA Tour players who otherwise didn't qualify. That's not the case at the Masters.