They were usually allocated at 1 per company (although I think they often operated as a section together). In common with most armies both then - and now - the pioneers usually had full beards.
Their main tasks were building simple structures and knocking things down for example at Waterloo the pioneers from the British Guards brigade loopholed the walls at Hougomont and made built a fire step IIRC.
Often they would be supervising other troops in these activities as well.