# How many Critical Hits for Assassinate? [duplicate]

I am currently Playing a Monk 8/ Rogue 4 The Rogue is an Assassin. pg97 of the PHB says that for assassinate the move

In addition, any hit you score against a creature that is surprised is a critical hit

I attacked a surprise creature with my flurry of blows from the Monk giving me 4 hits. Since the passage on Surprise in chapter 9 doesnt specify how many attacks count for "surprised" my DM ruled that only the first hit was a critical. My argument was that Assassinate uses the word "all" for the hits.

So who is correct via RAW?