Complex lighting situations issues_light intensity decreases > number lights increases I’ll try to explain the issue as best as possible, I’ll attach some pictures to better explain the problem.
I have noticed that the more “lights” are introduced in a scene (both interior and exterior) the more the intensity of each lamp is reduced.
I’ve notices the same issue with HDRI images. Again, adding more sources of light into the scene, results in a decreased intensity of the HDRI image itself.
Clearly, the problem becomes extremely limiting especially in situations where multiple sources of illumination are required.
In the attachments I’m showing the aforementioned issue, as you can see the source of lighting (in this case a simple Point lamp) decreases its intensity as the number lights increases. On every picture I’m adding one or more lamps.
The default "Path Tracing" integrator is coded so that for each sample, it will bounce light from a randomly-chosen lamp.

My suggestion is that you change the integrator to "Branched Path Trace", which you can change from the Sampling sub-tab.

Using Branched PT will make cycles take bounces from every light for each sample.  This comes at the cost of increasing render times, but with the added benefit of reducing noise.

More info here: https://docs.blender.org/manual/en/dev/render/cycles/settings/integrator.html

EDIT: As a further note, Blender 2.79 will feature probablistic termination of lights based on contribution.  Basically, it will randomly skip sampling from lights if their contribution to a sample would be below a certain threshold, causing it to spend less time computing bounces for lights which make little or no difference for a given sample.  This provides speed gains in complex lighting setups, particularly where some of the lights are distant or occluded.  More info here: https://developer.blender.org/rB26bf230