I think when the church is raptured to heaven it may be a fairly insignificant event. The tendency is to think that the church will continue to grow and grow and then when its taken it will be devastating for society with all these people missing. I think the opposite. I think there will be relatively few true Christians on earth when that time comes.
I don’t know. One estimate of born-again Christians in the world is about 1/2 billion (http://www.888c.com/worldChristianDenominations.htm), and that’s just church members.
The Bible says the number in Heaven will be “ten thousand times ten thousand, and thousands of thousands” (Rev 5:11).
I tend to think that many will be surprised at how many will be in Heaven, because of the miracle and power of the gospel of grace to save “to the uttermost” simply by calling on his name.