Distribution of tests across test agents
The default bucketsize (no of tests assigned to each agent per run) is 100. So if there are 400 tests and 6 agents, 100 tests each would be given to 4 of agents. The last 2 agents would be utilized. Reducing the bucket size is the option to utilize all the test agents in an environment. The option though is not exposed in the testsettings UI in Microsoft Test Manager (MTM).
You can use the utility mentioned in the blog https://blogs.msdn.com/b/aseemb/archive/2010/08/06/update-testsettings-utility.aspx for updating this info. In case of non-TFS remote runs, the testsettings file can be edited in the xml editor itself.
Some more info on this issue,