I have a Panodia portfolio. The "standard" insert has twenty pages (ten double-sided leaves) ... and that is, more or less de rigueur for a portfolio.
I am constantly revising - replacing some with new work. This portfolio is 11" x 14" ... and all the photographs are printed on 11" x 14" paper with varying borders. The hardest part is selecting which - at first, no one has "enough" - then rapidly ... FAR too many.
A couple of suggestions, gleaned from observing people check my portfolio, in my *very* subjective order of importance:
1. I would strongly suggest that all images be either in "portrait" or "landscape" format ... even if it means wild mixing of images sizes. Nothing appears to be more annoying than having to turn the portfolio itself after the initial image - even once.
2. If you work in different "topics" - Nudes, Landscapes, Street Scenes, Abstractions ... It is a good idea to have a separate portfolio, or insert for each.
Saves a lot of wear and tear, and the hassle of taking photographs out and replacing them.
3. It may be advantageous to separate still further; one insert for black and white; another for color. That depends entirely on one's aesthetic preconditioning ( - translation: If it works for you).
All that applies to the portfolio. In visualizing the people who will be affected, one will do well to imagine that they are stressed - a *bunch* - and probably squeezed for time. As an "acid" test, imagine yourself in their position/ mode; Go though the pages, fairly rapidly, and try to estimate what the overall impression of your work will be - on THEM.
A Gallery exhibition is a little different ... the number of prints will depend, a great deal, on the space available. In operating a "Town Owned" gallery, I usually see a new exhibition every week. To me, the greatest "sin" is crowding - trying to stuff too many works on the walls in a "shotgun" approach. It is the hallmark of an insecure beginner (NOTHING wrong with being a "beginner" - but if you do not have confidence in your work, it is a good idea - for all intents and purposes, necessary - to fake it). I will ALWAYS advise every one to that an error toward the side of "not enough" is FAR better than having too many. Give the future viewers time - and an opportunity - to experience each image.
There is a web site that to me, is very useful in contemplating a gallery set up: [ www.demarchelier.net ] - go to "exhibitons and prtraiture. Patric Demarchelier is one of the great lights in photography, and in his site there are many views of "first water" gallery exhibitions.