I seem to recall reading somewhere that you were limited to only two missions at a time, but since many of the missions seem to not be completable when you receive them, this doesn't seem to make much sense. What are the limits in terms of taking missions? What happens if you reach that limit and then talk to someone that would normally give you a mission? Is there a way to review the missions that you are currently on?
You can take 2 missions at a time per area per timezone.
I'm not sure about what happens if you accept more than 2 missions i cant help you on that.
There is no overview of all the missions you are currently doing. But you can see all the missions you are doing in the area you are in now by opening the map for the current area.