How many instances can be joined with the same email ID?

@Gargron

@dearthofsid Unlimited. Instances don't share private account data like e-mail or password!
@Gargron thanks for the response. however, when I tried to sign up at a certain instance, they rejected my form saying the email ID is already taken. why would that happen?
@dearthofsid @Gargron first see the spelling. If the spelling of your mail id is not wrong, then see for forgotten password and see if you get any. If not, get to the owner of the instance and tell me. There is possibility that somebody may be impersonating you. Although how s/he got access to your mail id would be a question.
@Gargron I was able to reset the password and they are saying my confirmation is under review. So I guess the account got created but still it threw a validation error. Must be a bug in that instance?