A list of 12K GitHub projects with Truck Factor = 1
medium.com
medium.com
The reason why ~3/4 GitHub projects only have 1 contributor is because a lot of people use Github for personal projects and/or as a portfolio for recruitment. So lumping them in together with all the open source projects that do have a community massively skews the results to the point of rendering them meaningless.
If we're going to talk about "at risk" projects hosted on GitHub then I'd be far more interested to know about the size of communities for any project that has multiple contributors. Or better yet, the truck factor for a subset of projects picked based on their appearance in the wild.
The projects are sorted by star, it represents pretty well the size of the community.