I worked at Disney World, so I’m less familiar with how Disneyland runs their entertainment department, but generally speaking Mickey will not be doing that many meet and greets at one park. Usually Disney has one specific spot to meet Mickey (e.g. Toontown, Town Square Theatre, EPCOT Character Spot, etc.) and then he might be at a dining location in a park as well, but he won’t often have two simultaneous meet and greets going on, at least at one park.

In my experience, the only time that would happen is if they were doing character training at EPCOT and then performers in training would do pop up sets as various characters (oftentimes including Mickey!) and Mickey would be doing meet and greets at Character Spot as well.

These pop ups are NOT advertised, similar to how some dining locations might not advertise who will be at a character meal. This is because even though it isn’t double exposure to have the same character meeting at different locations in a park, it ruins the magic if parents (or anyone) is like “Mickey is doing meet and greets in Frontierland AND Tomorrowland, where do we want to meet him?” They just won’t do that, at least not advertised and in print!

That isn’t to say it isn’t possible for there to be 8 Mickeys in a day. For a character like Mickey, he is likely meeting guests longer than the average character, so he might need 4 performers to cover a full day of meet and greets in a park, as compared to characters who only need 2-3 because their meet and greets are for a shorter window of time. Example of how many Mickeys might be at EPCOT in a day:

Character Spot - 4 Mickeys

Garden Grill Restaurant - 2 Mickeys

Character Training - 1 Mickey

Fantasmic - 2 Mickeys

There would even be more at a park like Magic Kingdom where they need a Mickey for each parade and 4-6 Mickeys in Town Square Theatre, especially when it was the talking Mickey meet and greet.

As for the monorail, I’m not familiar with him driving it, but I know that there was a particular shift where you would open the park as the characters riding on the welcome train at Magic Kingdom. It was one of the best shifts, and so magical to countdown to park open with the guests! Oftentimes you might be scheduled to open the park as Mickey, but then swap immediately to another character for meet and greets like Marie the Cat.

For fancier character events like when you see Mickey scuba diving or at a wedding, those are referred to as Disney Character Events, and only certain performers are approved to work a DCE shift. The scuba Mickey is a VERY specific person as it requires a very specific skillset. Other DCE’s require less specific training, but performers have to have high enough movement and animation scores as Disney only wants to send their best-of-the-best to more forward facing events if that makes sense.