SQL TFS Visual Studio Solution: One Database or Multiple Databases
A solution is meant to cover multiple like projects together. So for example, I have a "Data Warehouse Solution" that has all of my SSDTs related to our data warehouse. I then have a different solution for my "RDBMS Project," with more projects inside of that. It is a logic grouping for user ease.

There is nothing stopping you from having one master solution with tons of projects, or one project per solution. 

EXCEPT

I found the need to have an SSDT for the database server itself, if you are keeping track of server objects created at the server level. For example, security objects. Then I found that you couldn't create a reference to the database server. You had to create a SSDT project just for it.