A lot.

The answers to this question are not readily available. There are a number of paid market research reports that you can purchase that will provide reputable numbers, such as IBISWORLD or from marketresearch.com. You might be able to get access to abstracts of one of those through a library.

One response to a similar questions, which had source reference links that are now not accessible, placed the number between 8–9 million annually. That was posted in 2003, which would make the data most likely at least 8–9 years old.