How many PPDT stories should I practice before attending SSB?

I don’t think the number matters as much as people make it out to.
A lot of candidates practice dozens of PPDT stories and still struggle during screening, while some candidates practice far fewer and do just fine.

The real question is whether you’re getting better at observing the picture and building a story under time pressure.
One thing I’d be careful about is over-practicing the same set of pictures. Sometimes candidates become so familiar with common PPDT images that they stop looking at the picture properly. If a similar picture appears in the actual test, they immediately start writing a story they’ve already practiced.

The problem is that the actual picture may have small details that change the situation completely. If you’re busy recalling an old story, there’s a good chance you’ll miss those details.

Personally, I also think PPDT practice should train you to observe, think, and write quickly, instead of training you to memorize stories.
And don’t forget that PPDT isn’t just about the story. Narration and discussion are equally important. A lot of candidates spend hours writing stories but very little time working on how they present and discuss them.