You can add up to 100 senders per API Key. A sender can either be a manually validated sender address or a sending domain.
The Sender Domains & Addresses page will list all current sending domains and (manually validated) sender addresses. The total declared senders is the sum of the domains and the sender addresses (regardless of status).
From the example above, this account above has 4 declared senders:
If you do reach the 100 declared sender limit, the ‘Add Domain’ and ‘Add a Sender Address’ buttons will be disabled. To continue adding senders, you can delete sender addresses you are not using or create a new sub-account

From the example above, this account above has 4 declared senders:
• 1 validated domains (*@mywebsite.com)
PLUS
• 3 manually validated sender addresses (maria@sample.org, luca@sample.org, jon@mywebsite.com)
If you do reach the 100 declared sender limit, the ‘Add Domain’ and ‘Add a Sender Address’ buttons will be disabled. To continue adding senders, you can delete sender addresses you are not using or create a new sub-account