By September 29, 1956, the Yankees had already clinched the AL pennant. Going into that day's game, they were nine games up on the second place Indians with just two games to go. In the National League, the Milwaukee Braves led by half a game over the Dodgers, with the Reds just two games back. So, when the Yankees took on the Red Sox on September 29, they didn't really have much to play for. They were already safely in the World Series and could rest players as the NL was fighting to the end.
The Yankees sent Tommy Byrne to the mound that day. Of the 37 appearances he made that season, eight were as a starter. While he pitched a lot of innings for the Yankees that season, he was by no means a regular part of the rotation. The starting lineup also included the likes of Yogi Berra, Bill Skowron, and Hank Bauer. But considering that season's eventual AL MVP Mickey Mantle didn't start, I think it's safe to assume that Yankees were taking it a bit less serious that day.
The Red Sox scored an early run before the Yankees took a 2-1 lead in the fourth. Boston tied it in the fifth. However, the Yankees took another one run lead in the sixth, after which they made mass changes. Byrne was taken out, as were Berra, Skowron, Bauer, Andy Carey, and Gil McDougal. All of those players were the regular starters at their positions that season, and all would be in the lineup for Game One of the World Series four days later.
With a second string Yankees' team now in the game, the Red Sox took a 4-3 lead against Johnny Kucks. Kucks made the second most starts for the Yankees that season, including a complete game five days earlier. However on this day, he came out of the bullpen, as he did in Games One and Two of the World Series. He then pitched a complete game shutout in Game Seven. Baseball is weird.
Despite having nothing much to play for, the Yankees used some big names off the bench late in the game. Enos Slaughter, who also would get the start in Game One of the World Series, was sent up as a pinch hitter in the eighth. His walk and a single by Irv Noren loaded the bases for George Wilson. Wilson had come in earlier in the game in the mass of replacements in the seventh. Even though the game didn't mean much, Wilson was not given the task of hitting with the bases loaded. Casey Stengel sent up Mantle as a pinch hitter. If you're using Mantle off the bench in that spot, they were probably hoping for him to at least tie the game. Which, considering that they just emptied the bench an inning earlier in a one run game is...interesting. Mantle drew a walk. That tied the game, which was probably not ideal for a team that was a few days away from playing in the World Series.
The game would go 13 innings with the Red Sox winning 7-5 in the end. That's not important, however. What is important is that in all the Yankees used 26 players in total in this game. Here is what that box score looks like (from Baseball-Reference):
A whole two people, Norm Siebern and Bob Cerv, played all 13 innings for the Yankees.
In the 2015 season, the Rockies used 30 players in a September game against the Dodgers. That is tied for the record for most players used by one team in a game. The Rockies used 13 pitchers that day. That's a lot, but it was September and with expanded bullpens, eh, that might happen in an extra inning game. What makes this Yankees' game so incredible is that they only used six pitchers. This wasn't 2015 with LOOGYs and using the bullpens to play matchups. They just used 20 people to pinch hit or come off the bench.
Another fun fact is that in that box score you can see that Don Larsen played in this game. He was used as a pinch hitter...and then didn't pitch. He had pitched the day before, but still. Larsen wasn't a bad hitter, for a pitcher anyway. In 1956, he hit .241/.294/.380 in 85 plate appearances. It's just that the idea of using a pitcher as a pinch hitter is very foreign to 2015 baseball, especially considering Larsen was in the rotation. He got the start in Game Two of the World Series just a couple days later. He also got the start in Game Five. Just over a week after pinch hitting in extra innings of a meaningless September game, he pitched a perfect game in Game Five of the World Series. Baseball is great.