From 'Transports' (
pg. 183, 8th edition rulebook):
If a transport is destroyed, any units embarked within it immediately disembark (see below) before the transport model is removed, but you must then roll one dice for each model you just set up on the battlefield. For each roll of 1, a model that disembarked (your choice) is slain.
The rule doesn't care about the number of units that disembarked, just the number of models.