How often can I put a message on somebody's pass?

Two rules decide it, and the Wallet screen says both under the message box.

• Ours: nobody gets more than one message from you in a day. While you hold credits from a credit pack, it is two.
• Apple's and Google's: up to 3 messages a day reach one pass. Past that, Google still puts the words on the pass but the phone does not ring.

People who have already had their message today are left out when you press Count them, and you are not charged for them. Send again tomorrow and they are counted again.

On a Google pass, the notice from Tell the phone when a line changes counts toward Google's daily number too. On an Apple pass it does not. See How changes reach phones: at once or at the next check.