Photos Slideshow won't display all images in an album

No, there is no predefined limit to the number of photos in a slideshow.

But you may have set a custom duration for your slideshow.

From: Create and work with slideshows in Photos on Mac - Apple Support

To change how long each slide appears, click the Duration button , then do any of the following (not all options are available for all themes):

Set the slideshow to match the length of the selected music: Select Fit to Music.

Set a specific display time for the slideshow: Select Custom, then drag the slider to specify how long the slideshow should play.

Set a transition for the entire slideshow: Select the Transition checkbox, then choose a transition type from the pop-up menu.

Set slides to fit the screen: Select the “Scale photos to fit screen” checkbox.

CaroleW1899 wrote:

Is there a limit to number of photos in an album that Photos can handle? Recently I am not able to view Slideshows from albums as they only display the first few images and then either quit or start over.

Create and work with slideshows in Photos on Mac - Apple Support