On June 24th 2020, Ole Gunnar Solskjær the manager of Manchester United made a quintuple substitution against Sheffield United. Manchester were leading 3 - 0 in the second half, and with the game in bag, Solskjær saw no reason to save a substitution for later on.
On a normal soccer game each team can have upto 3 substitutions. But FIFA has raised the limit to 5 for the post Covid-19 lockdown games since many consecutive matches are being played with little rest to wrap up championships, and players haven't had proper training camp.
I do remember seeing the rare triple substitution, or a double by one team at the same time as a single by the other, but can't think of any instance with more players subbed all at once of the top of my head. Of course in friendly games, each team is allowed more than 3 subs, having there not been any cap till a few years ago. In theory it was possible that a manger would've subbed half or more of the team at once, but it would've defeated the purpose of the friendly match to analyze the performance of each player as well as how well they fit with their surrounding teammates.
Seeing 5 players of the same team being subbed at once makes me wonder what's the most number of players substituted at once - not necessarily all belonging to just one side - in any top tier league or international play, including friendlies?