Len has all my archive stuff so with a lot of searching he could probably tell where and when the most backcasters were around. I would guess late 70's early 80's because Dave Docwra did well using it to set loads of records and win loads of stuff I think during the early 70's?
Over the years I remember casting with Hylton brighty, Ian Hunt, Howard Grey, terry Houseago, Gary Setchell [best backcasting technique ever no matter what anyone says], Alan Forder, of course the north easts very own Bob Scott and probably a whole host of others who's names evade me at the moment.
I think the most I can remember at any one event would be about 6-8? Colin Howlett would know for certain as he's been around even longer than me!