Final answer:
A purely competitive industry has a large number of sellers, while other market structures like monopoly, monopolistic competition, and oligopoly have a progressively smaller or decreasing number of sellers.
Step-by-step explanation:
A purely competitive industry, also known as perfect competition, consists of a large number of sellers. This indicates an industry where firms are producing essentially the same product and no single firm can influence prices. As we progress towards other market structures like monopoly, monopolistic competition, and oligopoly, we observe a smaller or decreasing number of sellers. Monopolies feature a single dominant seller, monopolistic competition includes many firms with differentiated products, and oligopolies are characterized by only a few dominant firms.
In essence, the number of sellers in different market structures reflects the degree of competition, which affects supply and has implications on market dynamics such as barriers to entry and market power.