How can I make the best forms for my site? What should I ask? How many questions can I ask?
I dunno. It depends. Here is how you can figure out, though!
1. Require as little information as possible -- requiring info people don't think you need will lead to high rates of false info
2. Clearly mark what information is required and what is not
3. Fewer pages is not necessarily better -- use logical page divisions and be willing to experiment with them
4. Provide a prominent link to your privacy policy and explain in simple English what you will and will not do with the information
5. Validate early and validate often! Rather than letting users fill out a long form and then telling them all the mistakes they've made, use Javascript to validate on-the-fly. Here's a great tutorial on that:
Form Validation Tutorial
This may not be as specific as some people would like to get, but anything more specific will really depend on your exact situation. Most variables will have different 'ideal' values for different sites. The most important single idea is that you must be willing to test changes to your forms. Try different things and watch what happens.
Without too much effort you can have great conversion rates!