Yeah, as Tinkrbell said, it's one actor/actress per character. So if you see Mimi today and you see Mimi again tomorrow, she'll be the same Mimi.
That said, *some* characters have been passed on to new actors/actresses over the past few years. Specifically, Mimi Kaboom has had three different actresses. Mimi actress #1 changed characters to be Betty Shambles. That opened up a spot for a new actress to play Mimi (e.g., Mimi #2). Since then, both Mimi #2 and Betty #1 have moved on and there are new actresses playing both of those (e.g., Mimi #3 and Betty #2).
Other characters have left Hollywood, but have been replaced by their doppleganger. An early example was when Paige Turner first appeared in Hollywood, she was wearing the same clothes as a former citizen (I can't remember the name right now). Even right now, there's a citizen named Ace Victory who is dressed exactly the same as former citizen Holden Hollywood.
I say all that just in case you're looking at older photos or videos on YouTube and wondering why Mimi looks different now than she did a couple of years ago or why older pictures of Ace Victory refer to him as Holden Hollywood. In the case of Mimi, it's the same citizen, different actresses (although Mimi #1 and Mimi #3 look very similar). For Ace/Holden, different citizen and different actors, but same clothing.
