The price or "value" of each share of stock you own is very much reliant on how many shares of stock there are outstanding. If you own 1 share out of 10 shares issued, then you own 10% of that company. If the company issues 10 more shares, your share of the company is cut in half to 5%, and your value per share goes down (the stock price then decreases). Knowing the number of shares outstanding, especially relative to competitors in the same industry, can help protect your interests.