[Monsieur]

As some people already stated, the bigger search engines (e.g. Google) don't care about most meta tags (including the keyword-tag) anymore.

I myself am still doing all meta tags on portals or other sites where I'm mainly relying on search engines but don't include keyword-tags on my galleries or free sites anymore.

But I suggest that you should be doing it, because every visitor referred by a search engine could mean a sale, so if you don't want to throw money away you should include the keyword-metatag.

To the initial question:
Of course you can also include more than one word (e.g. horny milfs, milfs, horny, old milfs) in the keyword-metatag which is very common on the web.
The search engines don't have a problem indexing that.