You can associate as many Callback URLs as you like with your Client Account. Callbacks bear no relation to API Authorization Tokens.

Whenever one of the Event types occurs on one of the Transactions associated with your Blueprint Title Client Account, ALL of the Callback URLs associated with your account will receive a corresponding JSON POST Payload.

For most Clients, having 1 Callback URL will be sufficient. However, as mentioned, you may define as many as you like.