Industry structure is often measured by computing the Four-Firm
Concentration Ratio. Suppose you have an industry with 20 firms and
the CR is 30%. How would you describe this industry? Suppose the
demand for the product rises and pushes up the price for the good.
What long-run adjustments would you expect following this change in
demand? What does your adjustment process imply about the CR for the
industry?

The auto industry is probably a good model for your scenario.  40
years ago there were four domestic auto makers with one selling
greater than 50% of the cars.  Oligopoly right?  Is that true today? 
What adjustments could GM have made to retain a greater market share?