My personal take is three to five strong projects depending on your career path.
For the feature writer, two of those projects should be polished scripts with pitch packages to supplement. The other three+ can be treatments or professional pitch packages with a well-thought-out concept in which you would be confident writing the first draft in a month or two, if requested.
If you’re a TV writer, on the other hand, then you’re looking at three+ original TV pilots and maybe a spec script of an existing TV show. That spec is simply a way for you to show other writers that you can jump into someone else’s narrative and not blow up the place. You can capture the tone, characters and environment of something not created in your own head. Specs are not always required to get staffed on a show, but it can’t hurt to have it in your back pocket.
Finally, if you’re one of those writer/director types, then three+ of those projects should absolutely be short films to demonstrate your skills as a director. I would also suggest that any short script you have in hand is already on the road to production. Short scripts don’t do any good unless the produced film is right around the corner—at least that’s the expectation. So short films are a must along with a couple completed screenplays, and you should be ready to rock n’roll.
Ultimately, you want your portfolio to be a good demonstration of your skills and overall brand as a creator. When an agent or manager takes an interest in one of your projects, and naturally loves it, the next question out of their mouth is going to be, "What else you got?" Don't come without an answer to that question.
*Feature image by Fran_kie (Adobe)