There isn't a place to discuss panel widgets. Also, I couldn't find a very good list of KDE component choices. It appears to be only a few, until I figured that the search works as you type something in. But it is very slow and the words I would use are not recognized by the search engine.

Try these: desktop, panel, widgets

Please let us know when it is ready for primetime. Until then, just turn off the page or at least give a scroll bar to the complete list and users can do trial and error methods to figure the right category.

if you mean the guided posting, having a list entry for all possible panel widgets is close to impossible. The component you need for this is just "Plasma"

How many are there?

The problem is not the amount, but the hosting. Are they part of kdebase or are they hosted at kde-apps.org or are they on someone's own server reachable? Where can you find information? Who are the authors?
But even if not, technically, Plasma is the main component for all desktop widgets. Narrowing a forum even more makes it less likely to find someone with a good answer.