Well, if you get enough to test the most, then you can also test the least just by removing the appropriate number
There are three main issues that potentially get worse as you add more lamps on a
circuit: capacitive effects, initial inrush, and repetitive peak currents. Capacitive effects are mostly an issue with
SCR or
TRIAC dimmers because those need
voltage and
current to go to zero at about the same time every half-cycle. Capacitive loads mess this up by creating a
phase shift between
voltage and
current. Some dimmers allow you to adjust this "window" of time to mitigate the issue, but very capacitive loads may just not be fixable without affecting other aspects of dimming. The behavior you see when this happens is usually an occasional or frequent flash-to-full from a dimmed
level but sometimes it's bad enough that the
dimmer acts like a
non-dim. Sometimes you can fix this by adding a
dummy load (because it makes the overall
circuit more resistive), but swapping to a different type of
dimmer is always a better solution when possible.
Inrush and repetitive peak currents are mostly an issue for ELV-type dimmers (reverse-phase or phase-adaptive) since they generally aren't as robust as
SCR dimmers. These problems don't necessarily cause any dimming performance issues, but they can damage the
dimmer and may even cause premature failure of the LEDs. One extreme case I saw of the latter actually was on an
SCR dimmer: there were a couple of large chandeliers with something like 120 lamps on a single 100A
dimmer (retrofit from
incandescent lamps). In that case the owners were happy with the dimming performance and the dimmers were handling the load just fine, but they asked for help figuring out why they were seeing a lot of very premature
LED failures. What I found was a combination of repetitive peak currents in the lamps interacting with the big inductor in the
dimmer resulting in
voltage spikes of up to 480-500 Vp on a 120 VAC
circuit. That was surprising to say the least... but at least it clearly explained the failures.
There are also times where repetitive peak currents can more directly impact dimming performance with any type of
dimmer, but that interaction is kind of difficult to describe without more background information than I feel motivated to write at the moment. Suffice it to say that yes, too many can be just as bad or worse than two few when it comes to dimming LEDs.