Hello Catalin,
Here are the answers to your questions:
1. Is it possible to have that many?
[Antonio Chagoury] Yes, it is possible. There are no limits to the number of blogs you can have
2. How do I enable some kind of filtering/searching throughout the blogs?
[Antonio Chagoury]The Blog module comes with a "Search Blogs Module". Users searching for a term can chose which blog to seach from a dropdown menu.
3. Is it possible to for the bloggers to "tag" their post and then filter the blogs and posts by tags?
[Antonio Chagoury] Not currently. We are working on the next version of the module which will include tagging and categorization.
4. Is it possible to identify a blogger using their Username instead of BlogID ? eg: http://portal.unitbv.ro/Blogs/maican/15/Default.aspx instead of http://portal.unitbv.ro/Blogs/tabid/825/BlogID/1/Default.aspx
[Antonio Chagoury] Not out of the box.
Finally, the module is Open Source, meaning that you can improve/modify/remove its features based on your specific requirements. If you are a .net developer then chances are you can get this done. Alternatively, you can hire a DotNetNuke consultant to get it done for you.
I hope this helps.