Of course you can only select one (per IP address). It’s mapping an IP address to a name. It doesn’t make sense to have one IP map to many names.
It doesn’t matter.
As long as you have a PTR record and that record resolves to a name that resolves back to the same IP, you’re done with PTR! Don’t make it complicated. The PTR does not even need to point to a name you control. Many hosts have a default PTR record for every IP, and that’s also fine! People sometimes have cosmetic preference for a domain that is hosted on the system, and that’s also fine.
But, don’t make PTR records complicated. You need one, it has to resolve to a name, and that name has to resolve back to your IP. That’s it!