A rule of thumb is to have a maximum of 10-15 slides for your main pitch deck, excluding your appendix.
Keep it legible
Make it simple, easy to read and obvious. As much as your pitch deck should be educational and include data and information, it should all be portrayed in a concise manner.
Start with a simple font. Use large type, bold text and make sure there's good contrast from the background. Titles should describe what your slide is about.
If you include diagrams, the paths to the ideas on your slides should be straight and quick. You don't want investors to get stuck in a maze.
If you include graphs, it's best to have only one y-axe to avoid confusion. Your axes should be labelled clearly, using real numbers.
Explain anomalies in your numbers.