There are a couple of different ways to organize your custom tracking domains.
Based on per-business basis.
Use a single custom tracking domain for multiple sending accounts. This would make sense if your primary domain is old/warmed up. If you're running a lead agency, this is what we would recommend.
Based on the associated domain for the email account.
If you had [email protected] and [email protected], you may use inst.first.com for all email addresses associated with first.com. This approach would make sense when all of your domains are new, or if you're experimenting with a new campaign that could potentially have a negative impact on your reputation. By creating separate groups, you can minimize the damage from a bad campaign.
Note: For Done-For-You or Pre-warmed domains, we will set up the CNAME record and enable the custom tracking domain automatically.