Your galleries can have as many images in them as you would like. However, we recommend breaking your galleries up into different categories if you have more than 100 or so images in it. Splitting the images up into multiple categories helps your clients locate the specific images they might be looking for, and it will also speeds up the loading of your gallery.
The only place that we currently have a restriction on the number of images in your galleries is with the full gallery downloads. These are limited to 1750 images or less. If you have a gallery that has more than 1750 images in it the full gallery download options will be disabled.