Had an experience with a rental bike that raised some questions.

Rental was a road bike. I planned an ~50 mile ride just to see the country. About 15 miles into the ride a spoke on the rear wheel broke. At first I didn't know what happened. But when the brake started dragging I knew.

The LBS wanted me to ride it back. When I told them the wheel was so out of true it still drug when the brake was fully open they sent a van.

-Only broken spoke I've ever had. Best guess why it broke? Safe to ride the road bike? If so, how many can be broken and the bike is still safe?