It's a 1:1 association. When I changed banks and tried to sign up online, it kept giving me an error. I stopped by a branch, and my old bank had registered me at some point. The teller had to call support and remove my previous association before they could create the new one.

Also, the association is to receive, not to send. Sending is initiated by your bank, which obviously already knows your account number. You send from your bank account to an email or phone number, which Zelle then translates to the target bank account.