My kids at a very young age asked why there were Santas in every mall, as well as every parade, etc. We simply explained to them (and at the time no one had mentioned this to me, I just winged it) that Santa is so busy getting ready for Christmas, there is no way that he could do all the other things that people want him to do, so he asks his friends to help him out. This was really helpful too when they caught glimpses of many "Santas" together in movies, like Jingle All The Way. This carried over to characters like when we went to Disney, or even various Disney on Ice and other shows like that.
The caveat is that my kids and my friend's kids know that sometimes, the REAL Santa or princess or whoever will have a little free time, and will drop in to do their responsibilities themselves. So we make a game of trying to figure out if the one we're meeting is actually the REAL one!
As for having too many that people would realize there were more than one, you have Mickey in all these locations at the same time: MK (magician,) Epcot (m&g, and sometimes the Chase m&g, also in Garden Grill with C&D,) AK (in safari gear with Minnie AND in Tusker House,) DHS (sorcerer,) as well as Chef Mickey's, O'hana, and I think there's another that is escaping me now. And Chip and Dale are really EVERYWHERE, just like REAL chipmunks! LOL
And face characters are much harder to replicate. They at least have to strongly resemble the character they are portraying. They can just throw anyone into a Chip and Dale, Pooh or Mickey costume and teach them the mannerisms. Face characters also have to sound something like the characters they represent. Imagine if instead of the pleasant contralto that Elsa speaks with three octaves lower...what kind of uproar would that cause...I paid thousands of dollars for my vacation, stayed up until midnight to get my FP and Elsa sounded like a man....you get the picture.
Casting face characters is much more difficult than costume characters.
When we visited Epcot in September, we were pretty sure that the Belle we met at France was actually Bill on "her" days off! Beautiful, but just had that 'something' that made us think that. Oh, and the Adam's apple too! LOL
No ... it's not.
I'm standing in front of Fairytale Hall with a crying child ... a child who is crying because she's next in line to meet Anna & Elsa and her sister just texted her a picture of Anna & Elsa in the parade. The parade that was going on right at that very minute. And this other little girl is crying because she waited all this time and now thinks she has to wait until the parade is over to meet Anna. Did I mention that this little girl is dressed in a beautiful Anna dress herself? She's looking at Anna in the parade picture and I (the helpful CM) am trying to tell her that Anna will magically be in the room when she gets there and not to worry. And then the door to the room opens, and there's Anna, and now the little girl doesn't want to meet her because she can't possibly be the real Anna, because the real Anna is in the parade.
Or ... this one was fun ... kid walks in to meet Mickey Mouse at the Studios one evening. As he's meeting Mickey and getting his photo taken, I notice he's got his iPad and is on Facetime with a friend of his. In California. Meeting Mickey Mouse at the exact same time. Kid turns and looks at me and wants to know which Mickey is real, the one he's with or the one in California?
So yes ... people can be in two places at once, and kids will challenge you on stuff like that, especially the ones that are just at that age of "are they real or are they not real"?
Add to that someone going through their photo album after a trip and seeing their child with four different Elsas and three different Tianas. Each one having been introduced as the "only" Princess Tiana or the "only" Princess Anna.
That's how people know if A&E are in two or three or four parks at one time.
As someone else said, anyone old enough to be getting a text or facetiming should have the concept of "friends."
That's funny, because on our most recent trip, my husband and I agreed that the most beautiful female character we met (and we meet almost everyone) was Anastasia at dinner at 1900 Park Fare. Not so ugly!
I have seen some pictures of these stepsisters that take my breath away! I swear, it's the faces they make that make them appear "ugly," not their actual looks. And in reality, the things people do (facial expressions as well as their actions) is really what makes them ugly, not the way they look!