There is no documented maximum number of photos in a slideshow. It will probably depend on the amount of memory installed on your Mac, or the available storage.
But the most limiting factor for the length of a slideshow should be the patience of your audience - the goal should be to have only the best photos in a slideshow and to keep it short. The best slideshow is a short slideshow, where the audience will say "Can we see more? A pity that it is already over", instead of falling asleep, because they need a break. For travel photos I found that 50 photos are perfect, when shown to people, who have not been our travel companions, and no more than hundred photos, when shown to the travel companions. Less is More. And pick a theme without much animation. Many themes are cropping your photos and do not give the audience a chance to see a photo clearly because of the animation. If you have really outstanding photos to show, avoid the themes with distracting effects. They are only necessary, if you have only boring photos to show.