I've just planted 'The Sutton ' broad beans, says on the pack that they can be planted Autumn so hopefully ok. Haven't tried them before, but thought I'd give them a go, as I ran out and couldn't get any more 'Aquadulce', my usual overwinter variety, so now have 2 rows of each.
Wouldn't have thought you could put more than 2 or even 1 per bucket. Could be wrong but mine are spaced on allotment at 20cm (8"). Even at 15cm (6"), which you could get away with, might be pushing it a bit.