Walking around main street itself, I've only ever seen 1 current villager and 3 former villagers at one time, but there are always more hanging out in the shops and museum (I once found 3 of my current villagers in the various rooms of the museum on one visit).
- - - Post Merge - - -
p.s. if you mean how many former villagers can appear on main street in general, I usually see about 6 or so. Right now, I have Scoot, Anabelle, Ruby, Ed, Groucho and Camofrog hanging around for sure; I'll have to check whether anyone else is still hanging around.
6 villager in total can appear in Main Street itself. Villager can also still appear in all the buildings/shops but personally I've never seen more than 9 on my Main Street (on the street and in buildings)
the people on main street are the last 16 villagers uve had in ur town (including people still living there) so you can have 10 villager in ur town and 6 villagers who moved out in main street. or 9 villagers in villager, 1 current villager and 6 villagers who left on main street... If this makes any sense to u..