Hi,

My best guess for a rental bike is your are the rider that broke a Camels back.
Very likely heavy renters with poor technique give the wheels a real hard time.

Anyone that then comes along that can put some real torque through the
rear wheel can then break a spoke, though it never happens on their bikes.

Best tool for broken spoke is a spoke spanner and loosening the two either side.

How many can be broken and its still safe ? I'd guess 3 or 4 spread
around the wheel for a typical 36 spoke count, easy but very unlikely
on the front, and on the rear at that point IMO you really don't want to
explore how many further spokes you can lose without catastrophic failure.

FWIW with a spoke spanner and knowing what you are doing you could
theoretically lose half of your rear NDS spokes evenly and half of all
your front wheel spokes evenly (with assumptions) still being "safe".

That assumes none are damaged, whist the previous assumes they are.

rgds, sreten.