An oligopoly is a market structure with a small number of firms, none of which can keep the others from having significant influence. The concentration ratio measures the market share of the largest firms.
A monopoly is a market with only one producer, a duopoly has two firms, and an oligopoly consists of two or more firms.
There is no precise upper limit to the number of firms in an oligopoly, but the number must be low enough that the actions of one firm significantly influence the others.