Vintage Slideshow

I was creating a slideshow via iPhoto and have 700 pictures that i was using. When I selected Vintage a the theme, it reduced the number of photos that are already selected to about 600. I then did the same thing with 100 pictures thinking that 700 was too much. When I selected 100 pictures, it reduced to about 80......Why is this. How do I work around this. I was going to just create two slideshows with 350 or less. This is a mother day video I am making .......Thanks