Each order worth SAR 30 or more adds one slice to your loyalty pizza, and 6 slices unlock a free medium pizza that you choose. It is one slice per order no matter the total, so even a SAR 180 order earns just one slice rather than six.
Mind the expiry date: you have 30 days from completing the slices to use the free pizza. You can claim it at checkout, on the order review step. Website users find the loyalty program under account settings; app users tap the bottom-bar loyalty icon, select redeem and pick a pizza from the original menu.
Source: Maestro Pizza help center: loyalty program