well,
On the player side, it is virtually unlimited, as Slide Effect loads and cleans images between each slides.
On the editor side, it depends mainly on the amount of memory you have on your PC. But I think you can create easily a slideshow with more than 100 images… More than 100 is obviously possible but the user interface will become a bit slow.
In short, no clear limit: the more pictures you have, the more slower will be the interface (but the player will go full speed).