Does shock web damage stack?

Yes and no. Shock Web has a frame limit (24) on the number of times an enemy can be hit with attacks in a certain period of time. You won't hit more often, the damage doesn't stack, but you won't have to wait for Shock Web to be cast when the previous one runs out.

When an enemy is hit with Shock Web, they become essentially immune to damage for 24 frames. After that frame limit is over, they can be hit again by something else.

If you throw 50 Shock Webs on top of an enemy at the same time, only 1 Shock Web will hit every 24 frames.

The only benefit you get from overlapping Shock Web is to guarantee that there will always be a Shock Web available to hit an enemy in case the previous one expires.