The number of authorized shares is specified in the company's articles of incorporation. You can also see the number in the capital accounts section on the balance sheet.
Authorized stock refers to the maximum number of shares a publicly-traded company can issue, as specified in its articles of incorporation or charter. Those shares which have already been issued to the public, known as outstanding shares, make up some portion of a company's authorized stock.
Authorized shares are the total number of shares a company can legally issue, while issued shares are the number the company has issued to date. The number of authorized and issued shares may be the same or different, in which case there would be more authorized than issued shares.
There is no minimum number of shares that must be authorized in the articles of incorporation.