iKotomi
[10] Knight
How many does it take? McDonald's deliberately went this route even though the idea of cornering the hamburger market's absurd.
McDonald's introduced a system of getting people hamburgers faster and cheaper than anybody else, and had a business model that was better than anything else out there. Given that, they just produced to the point that maximized profits, and that was too much for other restaurants to compete with. Though this can hardly be seen as a bad thing, unless you were a competitor, or you just hate Mcdonalds.
Even then, McDonald's can't act as a monopolist, though they have a huge command over the market. If they increase the prices a non-trivial amount, people would just buy hamburgers elsewhere. The competition is better described as "monopolistic competition" because of McDonald's ability to differentiate their product as different from every other hamburger.
Firms will always try to become monopolies if possible, that's a given. Only with a monopoly can you achieve economic profit in the long run. But as long as the market has open entry, most firms can not actually exercise the monopolist's privileges (price setting), and would have to settle for product differentiation at best.
Microsoft has attempted the same thing, but the success of that is more due to the ignorance of the consumer. If the consumer is unable to recognize substitutes, then the model fails. This is sort of related to the reason why most monopolies in existence are the first firms to enter the market. Consumers tend to rely on their trust for brand name rather than actually investigate the alternatives that may be cheaper.