That's not completely accurate. Yes, in international play the limit is six subs. But middle blockers can be and almost always are subbed out by the libero, and subs involving the libero don't count against the sub count limit. So even in international play, middles play across the front, and then mostly do NOT play across the back.
(The one difference there to confuse Tim even more, in international play the middle rotating to the back row must stay in and serve. When their turn serving ends, then they come out and the libero comes in. So they play a "half rotation" in back. Under NCAA rules, one of the two middles must serve, but the other does not have to before they are replaced by the libero. And with the greater sub count, it isn't unusual for a team to 'burn' two subs on the count to bring in a 'serving sub' to serve for the 'serving' middle blocker.)
Between the greater number of subs allowed in the NCAA and the libero-middle subs not counting against that limit, the variations of how coaches choose to use those 15 substitutions varies greatly from team to team. When Wisconsin played Nebraska in the 2021 national championship match, it was Wisconsin who used more subs in the normal rotation, and ran out in at least one of the sets; Nebraska was using fewer and didn't run out. Yesterday, it was the reverse; Nebraska regularly uses more in 2024 and were right up to the limit when the second set ended.
(What happens if/when you run out? The six players on the floor must play the rest of the set, with the normal middle blocker/libero substitutions contunuing, because again, they don't count against the limit. But coaches are keenly aware of where they are with their count, and have "plan B" already in mind for what they do when they get close to and hit the limit.)
