How many locations can I set for the message customers get when they pass my restaurant?

It depends on your plan: one on Starter, three on Growth and seven on Pro. You set them on the Wallet & loyalty passes page, on the Card design tab, under Locations & lock-screen triggers, each with a short label, the coordinates and the message; the same rows are also under Settings → Branding & logo. Customers add your loyalty card to their phone, and when they pass one of the locations you have set, their phone shows your message. A second branch, a mall you deliver to, or the road your regulars drive home on are the usual choices. If your plan later changes, the extra rows are kept but only the first ones are used. The Loyalty & rewards tab in the docs explains the card.