When Mr Banks rips up the ad into little bits, he only tears it into 8 pieces, but then when they come flying out the chimney, there are lots more than 8!
The whole movie is about fantasy, it would not have been any fun without million pieces flying through the air, remember the movie is about fantasy and imagination. It was supposed to be that way.....
He might have ripped 8 times but look at it this way: if you rip a paper once you get two pieces, if you you rip both of the pieces together one more time you get 4 pieces, etc. Do you get my point