Hi Mike Heighway,
In the RenderSettings you can chose Multi Pass(side menu)>Multi-Pass(Setting)>Lights>Separate Lights> None/All/Selected. (See Image)
(If you like to have the three “qualities” of the light in one two or three passes is then the next options, but not your question here.)
If you have only the 50 then use “None” EDIT: all 50 needs then the passes selected in the Multi-Pass section, where you normally have the RGBA pass as well)/edit
If you have the 50 but some extra lights, then use “Selected” EDIT, only the selected ones are created in extra passes./edit
(The option “All” is excluded with your question already.)
So, if you have used the Selected, then all lights become grouped EXCEPT you switch the check box on the lights you like to have separated. See image. EDIT the 50 are NOT selected, the extra ones should be. As the is no Group option, the only way Group is in the three standard passes, that you have to add to the Multi-pass list. The checked ones “separate pass” will create with the Separate pulldown option create individual passes/edit
All the best
Sassi