I was told by a representative at Chase when I signed up for the united card that what happens is the credit card creates a AA account to deposit the miles in your name. What is suppose to happen is that AA is supposed to recognize you already have an existing account and deposit the miles there but if that doesnt happen you can get the accounts merged.

If you forgot your AA Advantage number you can log in with your ID and just by logging in you can see your number... write it down. Or use your email each time.