I'm not so sure for a make up artist portfolio, but i've learnt for a art/photography folio they want about 20 images/pages. Since i do photography, i took some pics in my uni studio for free. It is quite costly for professional studio, so i'd suggest taking your own photos.
- Like everyone would say, take photos in natural light. Avoid indoor lighting, since it can give your photos a ugly yellow tinge (unless you have a filter to correct it).
- If you use a 35mm camera you can get special portrait film with a speed of (ISO)100, that picks up colour as true to life and is fine grained so detail is picked up well.
- take a photo with and without flash. Then choose the best photo.
Generally, each photo should have different looks... be it
natural look/neutrals/ colour/ dramatic etc etc on a range of models.
I'm not too sure on sizings on photos... depending on the size of you portfolio (mines A1... but thats a art/photography one lols) Im assuming a make up one would be A4 (easy to lug about with you lols)
I'd have photos printed up at 9 x 6 / 8 x 10 mounted neatly in a porfolio.
hopefully this will help you somewhat... bleh this is all do it yer self folio talk... OR you can just forget what i said and get it all professionally done.. lols!