This depends on your lab setup. If you're willing to work in 30 day time segments. You can purchase the single license and not activate it. This will allow you to work temporarily for 30 days. The single copy can then be installed on multiple VMs simultaneously. Instead use the keys here just to put a key in: https://technet.microsoft.com/en-us/library/jj612867(v=ws.11).aspx
These will not allow for activation but get you through the installation. Then you can build your lab environment without activating your OS. You can extend this 30 day grace period longer (3x maximum) by using
slmgr.exe /rearm at an administrative command prompt. This will give you up to...180 days without having to activate with your licensed product key.
If you're building permanent machines that you intend to use and never destroy, then legally you should purchase the 5 cals.
**if the post above has answered the question, please mark the topic as solved.