Table: mission_heads
Columns: head_of_mission
tenure_begins
tenure_ends
french_head_of_state
japanese_emperor

Question: How many French heads of state have a head of mission of Xavier Daufresne de la Chevalerie?
SQL: SELECT COUNT("french_head_of_state") FROM mission_heads WHERE "head_of_mission" = 'Xavier Daufresne de la Chevalerie'