As vampires are fictional it would take an infinite number to kill 20 men. They would need to be hit by all the books or bored to death by them.
Five unarmed large men with good wrestling and kung-fu skills could kill a tiger.
Six averaged sized men with good strength that are good at avoiding the deer's hooves and antlers could kill a deer on the rut.
When they kill them and do not take care of their environment...
18 men.
24
Grendel is able to kill 30 of Beowulf's men in the mead-hall, Heorot.
30 FYI venom
six
you
24
Odysseus killed hundreds of men, both in war and at home.
men kill ermines for their fur