From API FAQ (it is worth reading):

Does the API key cost anything?

Our API is free to use as long as you attribute TMDb as the source of the data and/or images. However, we reserve the right to charge for the commercial API key in the future.

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What are the attribution requirements?

You shall use the TMDb logo to identify your use of the TMDb APIs. You shall place the following notice prominently on your application: "This product uses the TMDb API but is not endorsed or certified by TMDb." Any use of the TMDb logo in your application shall be less prominent than the logo or mark that primarily describes the application and your use of the TMDb logo shall not imply any endorsement by TMDb. When attributing TMDb, the attribution must be within your application's "About" or "Credits" type section.

When using a TMDb logo, we require you to use one of our approved images.

You can read more about attribution requirements here.

Is there a charge to use this API for commercial purposes? and if so, how much? Thank you

Hello @talestalker . Hope you are doing well. I'm a solo developer from India. I'm building a review site for movies, TV shows and books and planning to include affiliate links to the books on my site, which makes my usage of TMDB's API commercial. I have mailed sales@themoviedb.org for a commercial license and waiting for a reply. Please kindly advise on the pricing for your API. Thanks for your valuable time.