There are enough seats for everyone in each seating, though they may not all be together if you wait until 10 minutes or less before the show begins.
A few cruises ago there was a large family group who would try to save three rows of seats. Last cruise I mentioned what happened in front of us for two nights to the CM at the door. A couple girls attempted to save 12 seats. They were extremely rude to everyone, especially an elderly couple. One of the girls laid down across the arms of some of the chairs. They asked one of the waiters to help them save the seats and he just smiled and didn't say a word. This made me laugh and when the older girl looked back at me I told her sweetie, he's not going to help you save seats because it's against the rules to do that. He smiled a little bigger and walked away. The worst part was no one ever came to sit with them during that show. The CM was walking around the front and saw the girls trying to do the same thing. She told them they were not able to save the seats. The girls got up and left after calling someone on their wave phone.
ETA: The major vent I have about the theater is that the people behind us always seem to think that whatever they want to talk about is more important than anyone around them hearing what is going on on the stage. I find that extremely rude behavior towards the entertainers, and rude to those around them. Whenever the music gets louder, so do their voices. My kids have been taught how to behave when in any theater. It's sad when adults can't do the same.