If I was on the phone to my missus and said "I’m on the bus" she’d assume that I meant in it, not on top of it. difference in usage perhaps.

Accusation of chauvanism accepted, of course one of the girls could be driving the bus. Or in fact the bus could be parked and the driver (who could be any sex, race and persuasion you care to mention) might be off getting a sandwich

I have 2 medium sized cats and have trouble getting them both in my car. I really fail to see how you would get 7 big ones in a sack, of the ruck variety or otherwise, at least not one that anyone I know could get 7 of onto a bus.

James, how do you know one of the girls isn’t driving?

"As I was going to St Ives
I met a man with 7 wives.
Every wife had seven sacks.
Every sack had seven cats.
Every cat had seven kittens.
Kittens, cats, sacks and wives,
how many were going to St Ives?"

OK, here’s how I got my number (which sort of assumes minor trickery in the question)

7 girls x 2 legs each = 14
7 girls x 7 backpacks x 7 big cats x 4 legs each = 1372
7G x 7BP x 7big cats x 7 small cats x 4 legs each = 9604
1 driver x 2 legs = 2

14 + 1372 + 9604 + 2 = 10,992

or 10,990 excluding the driver,

although I could still call trick Q if the answer is, you can’t tell, because the other passengers aren’t listed...

etc etc etc