Forget monopoly, duopoly, triopoly, oligopoly and all the other opolies that some economists are capable of conjuring up.
Numbers are irrelevant to competition.
What matters is whether entry is free, i.e., there are no artificial barriers. If not then competition exists. Competition is not a datum or a numbers game it is a process, which also means that the mere threat of competition against a sole incumbent has the same effect as if competitors had actually entered the market. Try telling this to most economists. Professor Quiggin uses the neoclassical approach to argue that the Australian airline market is only big enough to support two airlines thus any so-called open skies policy will only lead duopoly.