If you have an Akismet Plus subscription, you can add sites to it at any time via your account dashboard. Because Plus subscriptions are per-site, you will need to update it as necessary to support the number of sites with which you would like to use the subscription.
Once you log in to your account and locate your Subscriptions section, click on the Edit link to modify your Plus subscription.
Use the Number of Sites drop-down menu to select the new number of sites that you need to cover with this subscription, and click on the blue Submit button.
You can use this to update your card on file and change the billing from monthly to yearly and vice versa.
Personal sites that would normally qualify for a free plan do not need to be covered by your Plus subscription. You can simply add a new Personal subscription to your account by clicking on the blue Add Subscription button on your account dashboard.
If you have trouble adding sites to your account or if you have any other questions please contact support.