If the listing is generic like "senior software engineer" and all of the requirements are generic like being able to program or do teamwork, it's probably a generic listing.
Applying anyway is a safe bet, but I wouldn't invest as much energy into tailoring your resume if it's a 6 month old posting from a 50 person company with a website that looks like it's from 2009. That company probably forgot to take the listing down.
> maybe you should say outright you're looking for 10 people
This wouldn't help because you have no way of knowing if they've already hired 9 people or 1 person.
The target can also change constantly. At a large enough company you continuously lose people for natural reasons, like retirement, moving for a spouse, moving to be closer to family. Past a tipping point you're just always hiring.