This may be a well trodden question but a quick search here on 'Multiple categories' got no hits.
How can I create a bunch of categories in one go? Is there syntax, such entering them with coma, semi-color etc that would allow that?
Right click on the parent category and from the context menu choose New Categories. It tells you in the dialog box how to create them.
If you're trying to assign (many) multiple categories to files, try using the Category Panel. See Help for more details.
When you type
Multiple Categories
into the Search field of the help, the first hit leads you to the corresponding topic in the help.
On that page, open the table of contents at the top. There, click on Creating multiple categories at once.