Is it "room(s)" or "room/s" when referring to the possibility of more than one of something? [Parenthetical plurals]

There are people who disagree with this but my interpretation is that brackets (or parentheses if you prefer) are for optional material, while a slash should be read as or (with a few exceptions in the form of established abbreviations). So the latter nonsensically reads "room or s", while "rooms(s)" means "room or rooms" and makes sense.

Here's a blog post on the subject which, without a link or more detailed attribution states:

The Chicago Manual of Style once answered a question similar to yours on its online blog. Here’s the reply:

“A term ending in ‘(s)’ is both plural and singular. If you must use such a device (and it can be a useful shorthand), you have to be prepared to adjust the surrounding context as necessary: for example, ‘the award(s) is (are) accounted for.’ A parenthetical plural verb must correspond to the parenthetical ending.”

This of course doesn't directly answer your question, but establishes the parenthetical plural as worthy of consideration, without mentioning the slashed plural.