There really are no official rules

There really are no official rules on the word count for a slide. Ultimately, you need enough words to make you comfortable delivering your message. Put enough there to serve as a mnemonic, but go for a very low word count. If you use a plethora of words, your audience will read the slide more quickly than you can explain it, making you strangely irrelevant to your own presenta-tion. One time, for fun, I delivered a presentation by just being quiet and letting my staff read the slides. It was a hoot. We were done at least ten minutes sooner!
The default template in PowerPoint is a slideument. This is not a visual aid; it is a document. Avoid two-line titles when giving a presentation because of the distance the eye has to travel across the slide. In fact, consider doing a presentation with titles only, like the one below.