Large number of sellers means that number of firm are large enough so that contribution to total output of the Industry by any individual firm is negligible. So, no single firm is in a position to influence the market Price on its own by changing its own output. Thus, Price remains unchanged.
Detailed Answer:
The implication is that the proportion of total output produced by a single seller is so insignificant that the seller cannot influence the market price by his own actions by changing the quantity of output it produces. He has no option but to sell at the price determined at the industry level.
