Xfinity comcast.net email transition to Yahoo email.

... Which of the six (6) email addresses will receive the notifications? ...

Perhaps all, perhaps one, perhaps none. I had 5 logins/comcast.net email accounts, and only got one "Changes are coming" email message (at my primary user's email address). Although that mail said "... In about 30 days, you’ll receive another email ...", I never got one. 

https://www.xfinity.com/support/articles/yahoo-email-migration-overview says:

How do I upgrade my comcast.net email account to Yahoo Mail?
Once you’re eligible to upgrade your comcast.net email account to Yahoo Mail, you’ll see a prompt to accept Yahoo’s terms of service when you securely sign in to your email at connect.xfinity.com. If you have more than one comcast.net email account associated with your Xfinity account, you will need to sign in to each email account and accept the Yahoo terms, as each mailbox will be treated separately.

See https://forums.xfinity.com/conversations/email/68e5cc7e19d01b57e46b0864?commentId=68e64c7119d01b57e46df868 for an example of what that will look like (unless they change it again). Also see https://help.yahoo.com/kb/new-yahoo-mail/SLN36803.html.

And if you want to use the Comcast-at-Yahoo email addresses with an email program or app, you'll need to use the Yahoo server settings. After you've accepted the Yahoo terms and have successfully signed in to your Comcast.net email at login.yahoo.com at least once, see:

• Yahoo IMAP server settings 
• Yahoo POP access settings 

(Edit to fix typos)