Spoilers from the opening scenes of the Last Jedi:
In the beginning of the Last Jedi, Poe convinces Leia it's worth trying to destroy the Dreadnaught. They launch several (10s?) of bombers at the Dreadnaught. We see they end up all getting blown up, which doesn't end up mattering because apparently only bombs from a single bomber were required to destroy it.
Did they launch so many bombers initially because they only expected one to make it? Or did they get lucky that apparently the Dreadnaught was lined with explosives to cause a chain reaction when the the successful bomber dropped its bombs? (ie they expected it to require bombs from multiple bombers)