In the deals section of the app, you get a new coupon every week for a $1 sandwich. So, you can get one per week or day depending on which coupon you're using, which can be scanned in-store or used on a mobile order. The $1 sandwich offer is good on almost every sandwich McDonald's offers with the exclusion of their Signature Crafted Recipe sandwiches and the Double Quarter Pounder with cheese. That means other favorites like breakfast sandwiches and Big Macs are on the menu.
That's a lot of dollar sandwiches. No one is recommending you eat a 563-calorie Big Mac every day through the end of September. That would be insane. However, you'll have a tough time finding a sandwich for less than that unless you go spend some time at grandma's house.