My bones say - close enough to 40 in round numbers.
Never played myself, I was 17-18 when I first heard of it. It appeared to me to have nothing to do with motorbikes or girls so it failed to hold my attention. We did however as younger teens play a (nameless) game that used a die, a ruler, an eraser, some pencils and a roll of wallpaper or similar.
The idea was to set up a battle, drawing in pencil the units of your army on your end of the paper. To move a unit you erased and redrew it, the dice determined how far a unit could move (in inches). To shoot you put the pencil point on your unit, put one finger on top of the pencil to hold it upright, and flick it with your fingers on the other hand. The pencil would leave a line on the paper which represented your bullet. Other than that there weren't any fixed rules, you could make up your own rules for each game...how small can a unit be, how many hits could a unit take, different coloured pencils for different bullets, a bouncing bomb is a series of pencil flicks, etc, etc. Had just as much fun playing that as a kid as I do playing WoT as an grandfather.
Have no idea if the game has a name or where it originated, it seems to be quite old, my dad showed us how to play but it was not his invention since I found other kids at school who had learnt it from their dad, not sure but I think dad played it as a kid in the late 30's, early 40's. While on the subject of kids games, here's
something a bit geeky that will blow a grandchild's mind.