You can add multiple lights to one LightSelect - you group them by how much control you want over them. So for example if you have three spotlights in a room, and you want to adjust their intensity and color in LightMix all at the same time, you can add them all to the same LightSelect. If you want to adjust them individually, you'd add them to separate LightSelects.
LightMix shouldn't have a significant effect on render times, but it does affect memory requirements - and if that takes you over the limit of the memory your machine has available, then it will affect render time, but not because of the LightMix itself as such but because the machine has run out of memory. (This is another reason that may play a part in how you decide to add lights to the LightSelects, as each extra LightSelect will require additional memory, so you may decide to group some lights into the same LightSelect, to give up a bit of individual control but reduce memory requirements).
I hope this helps!
