If I buy Unity Mobile, Unity Express 1/5/10/30/100/500 sites license - how many servers can this be installed on?
Unity Mobile is licensed per Mobile Site. One license for Unity Mobile can be installed onto one Plesk server only.
A Unity Mobile license will allow you to optimize up to 1/5/10/30/100/500 sites to a mobile-ready version (depending on the license value) on the Plesk server the license is installed on.
Should you need to optimize some other websites that are hosted on another Plesk server to a mobile-ready view, you would need to purchase an additional license for Unity Mobile for the other server as well.