Overall in that bus there's a total of 8 people: 1. The narrator, 2. Harry the husband, 3. Teenage girlfriend, 4. Teenage boyfriend, 5. Lip filler woman, 6. Phone guy, 7. Bearded guy, 8. The "driver"

So, after the bus crashed, the imposter got inside, got rid of the real driver's corpse and put it out there in the snow, went back into the bus, counted how many passengers were there, saw that it was 7, and then when everyone was awake he pretended to be the driver and basically lied to them about the number of tickets with the intention to make the passengers think the culprit was one of them.

If you were referring to the title, it said five other passengers, meaning excluding herself, so allegedly six initially (because that's what the fake driver told them which made it believable at first since none of them would have counted how many they were in there beforehand.)

Think imposter was just trying to turn passengers against each other to make himself less suspicious