At the Masters, every caddie has a number.
Augusta National Golf Club thinks the number so important that they print the caddie number of each player on the tee times and pairings sheet each day. The club does that so patrons can easily identify a player they're following if all they can make out of the player-caddie combo is the caddie number on the classic Masters caddie jumpsuit.
However, there's a system which determines how a Masters caddie gets a certain number for their jumpsuit. It's a simple system, but it works.
The caddie for the defending Masters champion gets bib No. 1. That means, in 2019, Patrick Reed's caddie and brother-in-law will be wearing No. 1 on his jumpsuit.
After the defending champion, the caddie numbers are handed out in the order by which players officially register to compete in the tournament. Since the Masters is an invitational tournament, there are no alternates and there is a defined number of players competing in the event. So, when a player arrives at Augusta National Golf Club for the first time during Masters Week, they register to indicate they are playing -- and they don't have to register, technically, until just before their Thursday tee time -- and are assigned a caddie number based on the order in which they register.
In 2019, there are 87 players in the field. So, there are 87 caddie numbers. Curiously, however, Corey Conners, the last man to earn a Masters invitation in 2019, doesn't have the last caddie number. That number belongs to Matt Kuchar, who has been in the tournament since last fall. Order of qualifying doesn't matter at all.