If no creatures have died by the time your end step begins, the ability of Feast of the Victorious Dead won't trigger at all. Provided the ability does trigger, it will count the number of creatures that have died as the ability resolves. In other words, creatures that die in response to the ability will count as long as at least one died previously to have the ability trigger in the first place.

The ability doesn't target any creatures. You choose how the counters are distributed as the ability resolves.

The ability will count any creature that died, including token creatures. For nontoken creatures that died, it doesn't matter if the cards are still in the graveyard.