Who can I send notifications to?

Only people who opened your card on their phone and allowed notifications from it. Nobody else: there is no list to buy and no way to type in a number.

The Phones you can reach count at the top of Notify shows how many phones that is. The same number is Push Allowed on People.

A phone leaves the count when its owner turns notifications from your card off, or the first time a message to it cannot be delivered, for example because they cleared their browser or removed your card from their home screen.

People who only sent you the Stay in touch form are not on the count, unless their phone allowed notifications too.

In one day, each person gets at most one message from you, or two while you hold pack credits. See How often can one person hear from me?.