About how many people would you say were in Jerusalem on the day of Pentecost?  (In comparison to how many people were added to the church).

There is no way to tell, but just to give you an idea, the Jewish historian Josephus, made this observation about the attendance at the Passover feast during the days of Nero:

And that this city could contain so many people in it is manifest by that number of them which was taken under Cestius, who being desirous of informing Nero of the power of the city, who otherwise was disposed to condemn that nation, entreated the high priests, if the thing were possible, to take the number of their whole multitude. So these high priests, upon the coming of their feast which is called the Passover, when they slay their sacrifices, from the ninth hour to the eleventh, but so that a company not less than ten belong to every sacrifice, (for it is not lawful for them to feast singly by themselves,) and many of us are twenty in a company, found the number of sacrifices was two hundred fifty-six thousand five hundred; which, upon the allowance of no more than ten that feast together, amounts to two million seven hundred thousand two hundred persons that were pure and holy; for as to those that have the leprosy, or the gonorrhea, or women that have their monthly courses, or such as are otherwise polluted, it is not lawful for them to be partakers of this sacrifice; nor indeed for any foreigners neither, who come hither to worship.

The Passover was the main feast of the Jews, so the feast of Pentecost would have had far fewer people, but it is apparent that Jerusalem had the ability to hold a large number of people for short periods of time.

See also:

Questions and Answers regarding the History and Geography of the Bible