Multiple domains from one admin

Hi there,

I’ve want to organize all the content of multiple domains from one WordPress back-end.
Today I found the plug-in MU Domain Mapping from what I understand it redirects subdomains / folders to the domain with a 301.

Does this affect Google rankings? Can I edit all the content from one back-end? Some experiences or other suggestions to manage content from multiple domains from one back-end?

Also is there a limitation on how many domains can be in a network? Or do this completely depends on the server? What do I need if I would like to use around 20 domains? Many traffic?

Thanks for the help in advance.