Hello, I'm considering using wordpress multisite for any network of 30 blogs.

The questions is: every blog features its own directory, files etc.

With multisite and domain mapping, everything will end up virtul, is great for search engine optimization?

Second question: that u think ranks better, an easy html search engine optimization site or blog?

For those who have good content Search engine optimization does not matter. However, that's not saying it ought to be overlooked. You should use WordPress MU with either subdomins like myblog.example.com or folders like example.com/myblog I favor subdomains but both of them are acceptable for Search engine optimization. Another option (and seems like what you look for to complete) is map each blog to some domain for example myblog.com and myotherblog.com that is fine too. I believe the primary factor is if you are using domain mapping each website is being seen as an different site rather than extra time from the original site (I possibly could be wrong here)

For the second question, both of them rate just like lengthy because the website is structured properly. I'll condition again that individuals offer much effort into Search engine optimization and never into produced good content that individuals wish to read.