Can I use Form Maker Pro on more than one web site?

All depends on what license you obtained, or want to purchase.
Single License allows you to make one installation on a single website and to process unlimited number of web forms.
Business License allows installations on up to 10 websites and unlimited number of forms in total.
World Wide License allows installing the script on unlimited number of websites owned by a licensee company. There are no limitations on number of web forms. Installing the product on websites other than owned by a licensee or reselling is strictly prohibited.
Under the ISP License one may install the script to unlimited number of websites. The license also allows installing the script to websites owned by clients of the licensee company.
Product reselling is strictly prohibited.