How many Ephraimites were killed at the fords of the Jordan in the Shibboleth episode?

Answer: 42,000

Anyone who said "Sibboleth" was seized and killed at the fords; at that time 42,000 Ephraimites fell (KJV: forty and two thousand).

“they told him, “Please say Shibboleth.” If he said, “Sibboleth,” because he could not pronounce it correctly, they seized him and killed him at the fords of the Jordan. So at that time 42,000 Ephraimites were killed.”— Judges 12:6 (BSB)