When purchasing stocks using a traditional brokerage account, the minimum amount of stock that a person can buy at once is one share, as stated by Investopedia, a website that specializes in investment information. Investors using a dividend reinvestment plan are able to use the dividends to purchase more shares and are not limited to purchasing a whole share.
For example, if a share of stock is trading at $20 per share and an investor receives a $4 dividend, the $4 can be used to purchase a fifth of a share of the stock.
Investors pay the same commission whether they purchase a single share of stock or multiple shares using a traditional brokerage account. Often, it is not cost effective to purchase a single share due to the fees.