You should have received an email, in which the mobile pass number is pretty clearly mentioned under pass details. It usually takes a few minutes for the mail to arrive. It should also appear in your account on the site.

It depends what you mean by number.

All you need to activate your pass and to add it to an account on the interrail/eurail website to make reservations is the alphanumeric code sent to you in your order email.

A pass cover number that some external booking sites ask for is not included in the mobile pass but can be generated using this tool.

Note: It’s best not to add to the app or activate passes in advance as they cannot be removed from the set device by users.