The SQL 2012 licensing reference guide available on the Microsoft site has this to say:

The SQL Server 2012 Developer Edition is a full-function version of SQL Server software—with all the features

and capabilities of the Enterprise Edition—licensed under the Developer Tools model, which is a “per user”

model. One license is required for each person that accesses or uses the software.

When using SQL Server software for development, test or demonstration purposes, only the users are licensed

and a corresponding license for the actual server systems running SQL Server software is not required. As long

as only licensed users have access to the software, customers can install as many copies of the software on any

number of servers that are employed exclusively for this use.

Seems pretty clear to me--you have to buy one copy of the Developer Edition per developer or tester, and you can then install it on as many servers as you like provided only the licensed users have access to the SQL server instances on those machines.