In Major League Baseball, four umpires are assigned for the game. In the event of an illness or injury to an umpire, are there replacement umpires at the game, or, can the game continue with just three? (Or two, for that matter).
They’ll go with three. All of the umpires came up through the minor leagues, where games are worked by fewer umpires, so it isn’t unfamiliar to them.
Three is normal in the minors (at least AAA, I’ve never seen anything lower.) I have seen two umpires work a game in the minors, with one behind the plate and the other roaming the infield, but generally behind first or second, depending on the situation. Not great, sure, but workable. There have even been games (though it’s been a long long time) with a single umpire, calling balls and strikes from behind the mound. I couldn’t do it, but maybe it’s easier live and with training than just by watching the pitch from the same basic perspective on TV.
I’ve seen A ball with two umpires as well. I’ve seen major league games when an umpire was hurt and they continue with 3. Never seen two umpires injured in a game.
That’s a switch 
In 1997, umpire John McSherry, working home plate, suffered a fatal heart attack. Just before he collapsed, he motioned for the second base umpire to come behind home plate, as the rules stipulate.
The other umpires were willing to continue the game but the teams did not want to keep playing.
I believe in most single umpire situations, the practice is to call balls and strikes from behind the catcher with no runners on and then move in to the diamond when a runner gets on. An other alternative is to ask each team to supply an umpire for first and third base.
As much grief as they take, from the umpires I’ve met, there are no greater fans of the game than those that stand behind home plate.
Officials in most sports learn different mechanics for different numbers of officials. The NFL uses a seven-man team, but, say, in high school, varsity usually gets five and JV four, and each position has different responsibilities depending on team size. A two-man team of basketball refs rotates differently than a three-man team.
Forgot the point…
In any case, there are mechanics for changing to a lower-numbered team size in the event of injury or whatever. Football umpires get slammed into and tackled a lot, back judges pull muscles, etc.
They only need one. AAA assigns 3 umpires to each game, and I think the lower level leagues all use 2. According to the rule book:
Note that, in the rules, a “championship” game is one that counts for the standings, not necessarily a playoff game.
I don’t think the umpire (at least in the professional leagues) can call upon anyone else to cover the bases or the foul lines, based on 9.01 (a). Assignment of the umpires is up to the league, not the umpire.
When I was growing up, three umpires were the norm in the major leagues. They played at home, first and third. With a runner on first the third umpire moved over to between second and third. In any case, they positioned themselves according to where the base runners, if any, were. World series games used 6, but I don’t think I ever saw four in the forties or fifties.
Incidentally, they wore woolen suits, ties, and hats and a great many games were played in the daytime, under full sun. Day doubleheaders on Sundays, Memorial Day, July 4, and Labor Day were ubiquitous and the same crew played both games.
Incidentally, McSherry died before the first pitch (on opening day, yet). Once a game starts only the umpires can decide to suspend play.
Found this at Baseball Prospectus. It summarizes games played with various numbers of umpires. The timeframe isn’t given in the article (or it might be, but I’m not finding it), but I think it’s basically “all of them.” The full article is part of the series they ran called “Watching the Detectives”, about umpires and umpiring, and worth reading, as is much of Baseball Prospectus, and worth the subscription fee.
#Umps Games % Last G
0 1,097 0.6% 7/12/1979
1 23,751 12.6% 7/11/1923
2 30,318 16.0% 5/10/1979
3 28,904 15.3% 8/24/2006
4 104,746 55.4% 10/01/2006
5 185 0.1% 6/14/1993
6 6 0.0% 10/04/1999
Total 189,001 100.0%
The game with “zero umpires” on July 12, 1979 was the second game of a doubleheader between the White Sox and Tigers that was forfeited to the Tigers after the ill-fated “Disco Demolition” riot between games.