EDIT 02: Here is the updated main illustration including most of your feedback. EDIT 01: Thanks for your great feedback! This is exactly what I needed, it's hard to know some of the details when you're not a biologist. I will update the design here soon based on most of your suggestions...

As many as you want. There's no limit beyond the actual number of the sites we have, and even then you could subscribe multiple times for the same newsletter with different email addresses. I don't know why you'd want to subscribe to 140+ newsletters, but hey. It's your inbox. :)

The title of the page when viewing the tag pages is not terribly useful, it should say something like "new java questions", not just "new questions". Now all my tabs have the same title ! With the old UI, the title of the page included the name of the tag.

I haven't checked the specific issue, but be aware that pins are auto-removed after 14 days, because we found this feature to be abused too much. If you feel so strongly about a particular message, why don't you just post it again and pin the new one?