How many customers can one message on the card reach?

Up to 500 cards at a time, and one message a day. On the Broadcast tab of the loyalty card screen under Marketing you pick who gets it, and the live count tells you how many cards match; over 500, the 500 oldest receive it. The message is 90 characters, one offer, and you must pick the date it ends, tomorrow to 90 days out, so it removes itself from the card that day. It is free and it never counts against your weekly WhatsApp cap. The Loyalty & rewards tab in the docs has a guide called Send a broadcast.