I use w3_total_cache plugin. But i can`t understand what is going on sometimes. I have enabled database caching and debug info at the bottom. Sometimes on the main page 90-96 db queries, sometimes 495-501. How can be so different results for the same page with the same parameters? Or how can be so much queries sometimes ? What is this meaning? Please explain me:)
Probably some code on your site caches the result of an expensive operation. When the page is cached by W3TC, if on the very same page generation, cache was being regenerated, you will see the actual queries that operation requires. After that, queries are like the usual page load ones.
Don't worry about it until you see it too often.