According to page 3 of the licensing quick reference if you have datacenter edition of Sql Server (not Windows Server) you can run unlimited. With Enterprise you can run 4 Operating system environments with Sql Server installed.

In the case of SQL Server 2008 R2 Standard, SQL Server 2008 R2 Workgroup, and SQL Server 2008 R2 Web, if you license all of the physical processors you may run the software in the physical OSE only. In order to run the software in virtual OSEs, you will need to license each virtual processor individually as described below:

For any virtual OSE, you can calculate the number of Per Processor Licenses required for the SQL Server edition that you are licensing by dividing data point A (number of virtual processors supporting the virtual OSE) by data point B (# of cores [if hyperthreading is turned off] or threads [if hyperthreading is turned on] per physical processor). If the result is not a whole number, round up to the next whole number.