backing up multiple iPhones to one copy of iTunes

Yes. Each backup is kept separately.

It may help to reduce potential confusion later to be sure each phone has a different name. The name is changed on each device in Settings > General > About > Name.

GMTA 😁

Yes, as long as you have enough storage space on your computer. To make it easy to identify each backup, on each phone go to Settings/General/About and give each phone a unique name. You can be creative; mine is named “Don't Panic”, for example.