Do you have tips or sources on the internet that demonstrate strong qualities in an art design portfolio?

I’m wanting to build my portfolio and present in a professional manner.

What makes a strong art design portfolio. What are employers looking for?

I’m not doing anything architectural. Just photography, painting, graphic design, album art stuff of this sort.

How many pages of certain works should I have? For instance, if I wanted to demonstrate my photography skills for some company that wants a photographer/editor how many before and afters should I have?