In case you haven't heard it, the joke is, guy breaks down in the middle of nowhere, doesn't quite Die On The Moors but makes it to one of them rural pubs where there's always a fire and the same ten blokes and not much talking. Rings for a tow, gets a pint, eyes all over him but they're very polite and they don't Wicker Man him. Some time goes by, one of them stands up, says in a voice that carries across the room "Twelve" and everybody does a wee chuckle and goes back to their pints.
"That's weird" thinks the protagonist, goes back to his pint. "Twenty-seven" says this one lady off to his right, some giggles and blushes and elbowing. "Thirty" says a bloke with a beard and everybody roars, pointing, great big guffaws, apparently it's the way he tells them.
He asks the barmaid hey thirty of what, she says well it must look a bit weird to outsiders but since Dave here kept telling the same three jokes someone said one time "Oh that's a Dave Number Three" and it stuck, and then it got out of hand and we ended up numbering all the favourite jokes that everyone knows off by heart anyway and now it saves drinking time to just shout out the number. Give it a go if you want luv, you'll fit in better.
So our guy shrugs and grins and he takes a swig of his pint and shouts "Sixty two!" and the room just ERUPTS, people are falling off their chairs, slapping their thighs, wiping away tears, big red faces, holding their sides, the whole lot.
"Wow, what'd I just say?" says the guy.
Barmaid calms down enough to giggle "we've not heard that one before"