Board Thread:Support Requests - Getting Technical/@comment-14250-20180418082855/@comment-24142930-20180418090424

If you just want to edit the category box on one particular wiki, go to Special:MyPage/common.css on the wiki of your choice and add the code. If you want the code to apply to all wikis, go to Special:MyPage/global.css on community central and add the code.

Because you can't scroll down on the source editor page itself (only the editing area can be scrolled) even displaying as much as 10 categories may be too much. I'm not really sure how to set a cap though sorry.

this page may help you with categories. I've never used it before, but the magic word may help with arranging category names alphabetically.