51 Chapter 10. How Much Stock to Buy and How to Buy It In this lesson you will learn about the appropriate notations and instructions to give a broker to purchase stock on your behalf. Determining How Much Stock to Buy So far you know what stock is, you know what a brokerage account is, you have opened either your cash account or your margin account, and you've decided you are ready to buy some stock. Now you need to determine what size trade you wish to purchase. Careful attention and thought in this step can save you a lot of money. Plain English The size of your order means the number of shares of stock you wish to trade.