Just to add onto what @MarkCassidy said, if you have a Partner license for development that is separate from your client's license, the limits imposed by the clients license do not prohibit the developer from installing as many development instances as are necessary up to the limit imposed by their Partner license. In this case, a "development instance" is basically any instance not available for public access or client access.
When you install any instance, it must installed with the license that it applies to. Thus, if you install a development instance against a Partner license, it must be installed with the partner license and not the client's license.
The rule of thumb is that if the client can see the site then that site must use the client's license. As such, so long as the site is inaccessible to the client and the general public, developers may install as many instances as are necessary (up to any limits stated in the Partner license) without being in violation.
Again, like Mark said, there are further restrictions that aren't included here and this answer should not be regarded as legally binding in any way. This is merely meant to be a guide to help point you in the right direction. Please defer to and confirm all information with your Regional Sitecore Sales office or Sitecore Representative.