I do two per cane. I think display growers who aim purely for the very best flowers do one per cane and cut off all the tendrils and tie them in. In contrast for the cottage garden look of sweet peas scrambling over and covering the canes I think you need more per cane, perhaps someone can suggest the right amount.

I think 2 per cane is ample, if you stop them a couple of times they will be quite bushy, { I presume you mean 12 foot canes not 12" }

yes 12 foot Derek..thankyou both :)