Your Loyalistic plan has a limit on how many contacts you can have with that plan.

For this limit, only working unique email addresses are counted, thus any contacts who have marketing denied or where email has bounced back are not counted.

You'll see the notification once you exceed your limit. This is a soft limit allowing you to have a bit of headroom before you hit a higher hard limit where e.g. email recipients are cut off.

So you have time to upgrade to a plan with higher limit or cleaning up your contacts. Maybe you can delete contacts that have been unresponsive to your marketing. A softer approach is to deny marketing to them, this will preserve their data and history.

Please note, that the notification does not indicate whether you have hit the hard limit, so progress carefully if you are way over your plan's limit