I would venture that about half of those 100 are active. It seems that there are quite a number of characters who get approved and either never make an appearance in the tavern or anywhere else, or make one post in the tavern and aren't heard from again. Probably due to a number of things, the main one likely being a lack of action.
I think we can all admit that PbP gaming requires a certain degree of patience; I wouldn't be surprised if a lot of people go to try it out, and find that it's not fast enough (either in an adventure or waiting for one).
Of course, that's not to say that second characters crowding the tavern doesn't have the potential to scare off potential newcomers. But as long as DM supply remains roughly equivalent to adventurer supply, things should be OK.
Also, the sooner we allow second characters, the less of a shock it will be to the system. The longer we wait, the more people we are going to have in the community wanting 2nd characters. In other words, allowing them right now would mean the flood of characters is less than if we allow them, say, 6 months from now. In theory, at least.

It also sounds like there are a number of people seriously considering DMing in the near future, so perhaps this would be a good time.