First a question, where can I find the statistics to see prove for the fact that there are less posts when the Steem price decreases? I'm especially interested to see how big the difference is.

If you search Steemit for something like "price posts correlation" you'll find quite a few posts on the topic with graphs. I'm not aware of any Steem tools that show you both at the same time.

Second, I think we should all post less. Many tunes I've read the advice to write 1 post a day. But imagine what happens if you follow 40 people. Are you going to read 40 posts per day? And will you write a comment for each post?

There are arguments both ways here. I think the strongest argument for posting as much as possible is that it will result in more search engine traffic. If we bring more people to Steemit, it will increase the number of Steem accounts, the Steem network will be used more, and Steem should become more valuable.