Ten business-card-sized cards per Letter sheet. Print at 100% scale (no "fit to page" — it shrinks the QR's quiet zone and can stop it scanning), on card stock if you have it, then trim on the faint guides. One goes in every updater kit.
The QR and the printed URL both point at
app.florapulse.com/updater. Regenerate the code with
node firmware/field-flasher/tools/make-card-qr.mjs.