It’s complicated. I usually have two combat encounters of hard to deadly difficulty in each session, with a short rest between them. Any more than two combats (regardless of difficulty) and I get bored.
My dad typically runs three combat encounters per session. I don’t know if he uses the combat encounter difficulty chart in the dungeon master’s guide, but they seem less challenging overall.
The guy currently DMing for our home game tends to have only one combat encounter per session, but I would argue this is because he uses a battle map rather than theater-of-the-mind.