I do not think there is any real limit.
From the FAQ page:
8. I created a gallery with 300 images. When I view this gallery, my computer slows down or freezes.
SimpleViewer is designed for viewing at most around 50 images per gallery. More images than this can cause slow performance on older machines, as all the images are loaded into memory simultaneously. Try splitting your gallery into multiple sub-galleries.
I have galleries with 100 images that work fine.
Also I have gone as high as 250 without problems.
Gallery 1 Images 101, Image size: 120kb, Thumbnail size: 10kb, Total Galley size: 13MB
Gallery 2 Images 95, Image size: 85kb, Thumbnail size: 6kb, Total Galley size: 9MB
Important: to make files as small as possible use JPEG quality for images about 85%, for thumbnails use maybe 60%, and remove ALL EXIF data. Buildgallery.php will remove the exif data automatically when creating thumbnails. Small thumbnails is very important only use 65x65 pixels and remove EXIF data!
I have managed to get the thumbnails as low as 3 or 4 kb with good results.
PS Do not embed a color profile either.