thanks for the lovely comments!
it tooks 8 pairs of jeans.
my machine is actually a cheap and cheerful sort of thing. not that great but it coped well with just the denim. to put the backing on i used my mum's industrial one. i'm sure mine wouldn't have stood up to that. in places it was 2 layers of denim and 3 of backing. i'm pretty sure mine would have just melted into a plasticy mess from the stress of it all!
oh yeah and it'll be a normal double bed that it goes on.