Almost none? There have been a handful of mismanaged failures, but I can't think of a prominent successful scam. Dollar for dollar, probably 99% of Kickstarter backers of video games get a finished product.
Isn't that a bit like asking how many scams have been pulled off in dozens of segments of the $18.5 trillion US economy in general? Consider just the scale of real estate related fraud for example, whether on the consumer side or the commercial developer side etc.
The video game industry is very large. That there would be millions of dollars worth of scams in it annually (of one variety or another), would be entirely expected.
Making a good video game is hard. There have been several instances of Kickstarter-backed games not living up to the potential or being particularly disappointing, but this is also often the case for games funded privately by massive corporations or investors (often far exceeding the funds of a typical Kickstarter project).
Very few instances of willful scams, as far as I know. Probably not any significantly more than other kinds of Kickstarter scams.
