A number of these number plates are missing.

I'm not sure what you mean by "OK" here. OK for what purpose?

It's not a sentence I'd be please to write. Two reasons: first, "number plates" isn't clear. Licence plates? Something else? Second, it's ungraceful and rather boring to have the word "number" used twice in the same sentence and so closely together.

Why do you ask?

Echoing what blatham has written. Perhaps you can rewrite your words as Several of the number plates are missing

OR?

Several number plates are missing.