Varies from aircraft to aircraft but you may only have one exit row that actually gives extra legroom in the basic economy section on some wide bodied aircraft , so maybe 8 or 9 extra leg room seats for 160- 180 economy passengers...I"m not sure how you'd decide who gets those..and of course the airlines would like to be able to charge for their allocation...
I seen the routine calls have started for airlines to be banned from cramming people in but passengers like cheap flights...you can't have basic economy seating with pitches at 1960s or 1970s levels and also enjoy some of the current fares available.
Have to say you can sometimes use seatguru to spot the better seats (e.g. bulkhead) and of course there is economy plus...