A Portfolio is a representation of your work ... not - necessarily - the final product. Ar first it is a challenge to "get enough", but as time goes by, the real challenge is selection.
I have taken a hard look - and this continues endlessly, at mine, trying to place myself in the position of one choosing for a gallery exhibition. At times, that is NOT effective, especially if you find that you would be working far outside of your "style". I would suggest choosing a gallery that goes with your style, rather than the other way around.
After seeing **many** gallery exhibitions, I have concluded that the greatest sin one commit is "overpopulation". I've seen galleries literally "crammed" with works from floor to ceiling. Not good - it does not allow the experiencer time to absorb any of the emotional content that might be present.
It is FAR more preferable to err on the side of "too few".... although I've NEVER seen an exhibition that I could describe as "too few".
I've chosen to limit my portfolio to twenty photographs ... The Panodia Portfolio I use has a ten page insert - room for twenty - each back-to-back.
I may at some time purchase another insert - for a different subject grouping - but I'll never display more than twenty images at one time.