When I look back at my most successful slide decks there's a pattern, I call it the 2/4/8 rule: about every 2 minutes I have a new slide (about 30 slides for a 60 minute speech), no more than 4 bullets per slide, and no more than 8 words per bullet.
The 7x7 rule is simple: For every slide, use no more than seven lines of text — or seven bullet points — and no more than seven words per line.