No inclusive slideshow of a library file?

I am not sure what you mean by "file", as you are asking in the forum for the Photos.app.

In Photos you can play a slideshow as described here: Make a slideshow on iPad - Apple Support

Just collect all photos you want to include in the slideshow in an album.

Make a slideshow from an album or collection

1. Go to the Photos app  on your iPad.
2. Tap to Collections.
3. Open any album or collection, tap , then tap Start Slideshow.

I am using Apple's Keynote.app to make slideshows. Then I have much more control over the layout of the fotos on the slides and can add titles to the photos.

Apple does not document any limit the numbers of a photo in a slideshow. It may depend on the system version and the storage on your device. If you are planning to show your slideshow to others, I would not make the slideshow very long. Most people will easily get tired when watching a slideshow.

For a screensaver or photo frame you may want to have a random access to all your photos, but Apple does not offer that option for a slideshow.

I'm sorry, léonie-- this is so off the topic! But how do you load maybe 50 pictures into Keynote and get one picture per slide? I'm embarrassed that I can't figure this out. I ended up dragging them in one at a time from a Finder folder.