The React license is for one domain, therefore a new copy of React will need to be purchased for each site you use. Read the full license here.
Even with one license per website, React is still a great deal for developers. The cost of the premium plugins covers over 70% of the total cost of the theme: Quform – $25 and Revolution slider – $19. In addition to the high quality and extensive functionality available with the React theme, ThemeCatcher also offers free 5-star support which will give you peace of mind.