The number of domains for which you can configure DKIM is based on your plan. 


You can configure up to 2 domains with DKIM if you are on the Blossom or Garden plans, while on the higher plans, you can configure an unlimited number of domains with DKIM, as required. 


If you have been on the Sprout plan prior to January 2017, you will be able to configure up to 2 domains with DKIM. Sprout plan users who have signed up post-January 2017 can configure only one domain per account.