I know of two ways to learn the category #:
(1) Go to the category list (browse) page [for example, the eBay page that lists all the "Casino - Chips"; or "Casino - General"]. You can get there by a number of means. One way is to find an auction that was listed in that category. Then you click the link at the top of the page, just under the title of the auction and the lot #. That takes you to the category list (browse) page. (Or, get to the category list (browse) page by just clicking the category name on eBay's big list of categories page.) Then you click the link (in the upper left area of that page) that says something like this: "sell your item in this category." That takes you to the "Sell Your Item" (or a similar name) page where if you look hard enough (in the upper areas) you will see the category number.
(2) An easier way (for me) is this (with my Mac computer and Netscape browser): once you are at the category list (browse) page, look at the top of the window for the URL. It contains the category number.

There is probably an easier way, but the above works.......

Robert.