You can send one short notification a day, 4 in any 30 days, to the people who have bought from you. No links or contact details are allowed in it.
"Tell your supporters" is on your dashboard. It sends a short notification to the people who have bought from you — not to everyone who follows you.
The limits
1 a day, and 4 in any rolling 30 days. Up to 160 characters, so it fits on a lock screen.
They are deliberately tight. A notification from you lands with your name on it and gets read; one that arrives every day gets the notifications switched off, and somebody who does that is gone for every creator they support, not just you. The limit protects the channel you are using.
What cannot go in one
No links, no @handles, no email addresses and no phone numbers.
A supporter tapping a notification with your name on it is trusting you, and a link is how that trust gets used to move somebody to a site with no refunds and nobody to complain to. Anyone who taps yours always lands on your own page here.
If a message is refused, you are told why and it does not count against your daily allowance — so a rejected draft costs you nothing but the retype.
When it does not arrive
A supporter who has turned notifications off, or never granted permission in their browser, does not receive it. Your allowance is spent on sending, not on delivery, so it is worth writing something worth reading rather than sending one to test it.