The 2 / 3 / 4 / 5 target limit has been in Shock's description for quite some time now. Based of what I've observed since Volt's update in U11, I suspect these values are no longer correct. Just to clarify, this target limit is referring to the maximum number of enemies that can receive the initial 200 Electricity damage.
After a couple hours of testing, there still seems to be a target limit for every rank. However, the old values appear to be off by 1; Shock has a target limit of 3 / 4 / 5 / 6 based on my observations.
EDIT: The original 2 / 3 / 4 / 5 target limit does not appear to be a target limit at all. Rather, these values represent the number of chains Shock can make at each rank. As an example, for Shock at max rank, the electric arc can be redirected a maximum of five times. If Shock is initially aimed at an enemy, the arc can damage a maximum of six enemies. Here's a crude illustration showing why that's the case. See the comments below for more examples.
I also want to address a statement that was recently added to the description:
"Can chain off electronics, and deal extra damage, even if they're already broken."
I can't say that I'm observing this at all. Using Shock against electronics has no effect on the damage from what I've seen. Here are some examples:
If you're wondering why there is no secondary damage in the last screenshot, the initial damage was enough to kill the elite lancer. Is anyone able to deal extra damage with Shock by chaining off electronics? An explanation from the original contributor would be helpful.