It's not just one at a time imo, the logistics of it all are going to be very difficult to pin down without massive, heretofore unseen cooperation between all entities using them. Think of how many straight lines exist between a single Domino's and every house it can service, and imagine drawing that straight line on a map. Now do that for every single Domino's. Then do it for every McDonald's, every BK, every Wendy's, every DD, every Starbucks, and that's just a few restaurants. If this were somehow simultaneously implemented in every store and restaurant overnight tonight, there would be nothing but colliding drones all over the streets. We'd have to pin down regulation altitudes that didn't mix each other and be prepared to prove and fine any violators- since lower heights should be faster processes and thus entities will fight over those "rights." Delivery points would need to be slightly different down to a foot or two so a house ordering two things at the same time doesn't have them collide at the destination or land on each other. If someone decides to plant a tree in their yard, we need to hope the drone doesn't divebomb into it thinking that space was clear the last few times.

We could probably solve this with more technology, and communication between delivery drones- similar to how a city full of auto cars could have them transmit location information to each other to speed up traffic. However, similarly, that requires all the entities making the drones to play nice and nobody trying to exploit the system to gain an edge.