There is no limit to the number of images that you can have in a gallery but you might not be able to add all the images to JuiceboxBuilder-Pro at once.

Unfortunately, if you try to add many images to JuiceboxBuilder-Pro at once, then Adobe AIR (the platform on which JuiceboxBuilder-Pro runs) may not be able to allocate enough memory for the task at hand. Even if your computer has plenty of free RAM, Adobe AIR applications are limited to around 1GB of memory allocation (as far as I am aware).
The workaround is to add your images in multiple smaller batches. Trial and error will help to determine a safe number of images per batch (it will depend on your own images). From your notes, it sounds like you should be able to add images in batches of 100 (or less). Try adding 100 images in your first batch and then the remainder of 78 images afterwards.

I hope that adding your images in a few separate actions instead of just one is not too much of an inconvenience although I realise that it is not ideal.