Hi nicolearancibia,
This easiest (least technical and most modern) method is to add a 'User Update' canvas step after each of the '20 communications'. This 'User Update' step can be used to add/increment 1 to a custom attribute (must be a number data type e.g. name: "TotalSpecialCommsSent") on a user's profile.
You can then create a segment or filter directly based on the count of TotalSpecialCommsSent., i.e. TotalSpecialCommsSent less than 20
If you need to be 100% sure a user has received any of these 20 messages (and not just aborted/passed through the steps (for any reason)) before recording +1, you should add a 'decision split' step' inbetween, with (last received specific message step less than 1 hour ago). This way, users who do not receive a message will bypass the next 'User Update' step.
