Can you give us more info about the size of the space, how they will be displayed and also the size of photograph you can produce? If you have any images of the space it would be good too. These details are going to determine how many photos you need. Too few and the exhibition will look lost in the space, too many and your photos may be cramped.

30

I’m guessing people might want to spend half an hour to an hour with photos. Longer if they talk to the artist. You might spend between ten seconds and a few minutes with a photo, depending on how much you like it, but on average, lets’ say half a minute. Maybe another half a minute digesting or talking or wandering. So maybe a minute per photo. I’d say maybe 45 photos then. Although that seems a bit on the high side. So maybe more what @burntbonez said, although that might be a bit low.

It also depends on the type of photos and how varied they are in terms of viewing experience. I’m tempted to say less is more if you want people to experience each photo individually.

Space them out so you can view each of them at ideal distance without any other picture in the direct field of view… no distraction from each other. If any are meant to be seen together then that group should have its space without distractions.
Rank your pictures according to which you definitely want to include and some that are optional.
Given that, see how they fit the wall space available.
“How many” is just a count of what you have done above.

I don’t have an answer but good luck! And can we have a look too??