The number of DMARC capable messages in paid plans is the number of the email messages that meet ANY of these criteria:
1) sent from the user’s authorized IP addresses included in the user’s domain SPF record;
2) passed SPF evaluation;
3) passed DKIM evaluation.
It is recommended to choose a plan that covers the number of DMARC capable messages which is the same or is higher than the number of messages you send from all your legal sources per month.