Photos slide show

For playing in Photos there is no limit as far as I know. Efficiency in playing will depend on the amount of memory and the speed of the hard drive.

However for exporting as a movie file a lot will depend on the amount of free space on the boot driver, on the destination drive if being saved to an EHD and on the amount of memory in the Machine.

Some users have had problems with large slideshows not being saved or crashing Photos during export. How many are you wanting to use and do you want to export as a movie file?

I don't think there is a specified maximum - though you may get into memory problems with very large slide shows.

Try to find out how many photos you intended audience can stand before it is falling asleep. Fifty photos are interesting, hundred photos are o.k., but any more are very risky.