It’s impossible to say for sure, but plently. How many did God drown in the flood or burn to death in Sodom and Gomorrah? How many first-born Egyptians did he kill? There’s just no way to count them all.
But sometimes the Bible tells us exactly how many were killed by God. Here’s a list of those that I can find. (If you find any that I’ve missed, let me know and I’ll add them to the list.)
So far I come up with a total of 2,038,334 (not inlcuding, at least in some cases, women and children).
Dwindling In Unbelief: How many has God killed?
How many has God killed?
