Of the players that have gone, only McCann was a first-team regular last season. Also (as you noted), we need to add quality to the squad - not quantity - so your "we need to buy ten players" line makes no sense at all.
At the very most, I think O'Neill is looking for six players this summer - a keeper, two defenders, two midfielders and a striker - and if we get that many in, we'll have good cover for the season.