People I know say only 8-12 images, your best. The people who buy images are busy and just want an idea of how good you are and some of your best to judge from. Looking at more can come later after you become a known quantity or they reach the point of "Do you have... or can you do...?"
And you tailor the images selected to the potential customer. A car magazine wants pictures of cars etc.
If you give them too many, they have more of a chance to find something that turns them off. Think about images you would like to show and give you pride. 8-12 sounds about right. Show the portfolio to a friend familiar with your work for a reaction. Often we love something due to the effort put into it or some other emotional attachment that others just do not connect to. While other images that took little effort but create connection might get over looked.
DannyKayeLoc: Sheffield now but soon moving to Blanzay
We used to ask prospective students for 50 images but the lecturers rebelled as that was far too many. Go for 6-12 really good images. Get prints as they have more impact than an iPad screen.