There may be different market segments that could use your product or service. Think about all the possible customers for your product. Then research on the interenet to see how many there could be.
The U.S. Bureau of Census has all types of data for individuals, businesses and much more. Also look into trade groups that represent your type of business or the group you want to sell to. They will often have statistics on their members and other information that you may find valuable.
Post your specific questions here and also on your profile page under “What I need” and let our other members help you get your answers.