I don't know for sure, but I doubt they would have more than one of any given character out in a park at a time. Disney works very hard to maintain the illusion of magic, and having more than one Mickey wandering around would compromise that.
i'm not sure about disneyland but while we were in disney world during the christmas holiday, we saw hook and smee near the flag pole signing autographs and posing for pictures...as we walked further back towards the castle, we saw another set a hook and smee doing the same thing...the time difference between seeing them was not more then a couple of minutes and i'm sure that they didn't make it that far back that quickly...but as the poster above said, disney goes to great lengths to keep the magic alive....
sunking