In addition to YR's good advice, I would also look at this from another angle:-
1. How long do you think the audience will watch the slideshow before losing interest? Call this 'time1'
2. How long do you want each image to be displayed for? Call this 'time2'
As a first approximation :
Number of images = time1 (in secs) / time2 (in secs)
I totally agree with YRs comment . . . .'Better to use less than more, better quality than lower quality . . . .
For a slideshow of for example a trip, journey, event etc, use images which have a WOW factor, a different take on the subject (pun intended) and a suitable sound track / music / effects and you will hold their attention leaving your audience with a feeling of being immersed in the slideshow, feeling they were there and most of all impressed.
The last paragraph is very subjective but can be done.