We have tickets for the Audience with the Pope in October, but I have no idea of what to expect or even where it is held...do YOU know how many people will b there normally?
If the weather is not so nice or colder, the audience is held in the Paul VI Hall which I think is to the left of the square if one is facing the basilica. In any case, you should probably scope it out beforehand so that you know where to go Wednesday morning.
There are usually at least thousands of people, probably more in the warmer months. The square can hold upwards of 100,000 people. They do have barricaded areas so that the Popemobile can drive around.