I can remember reports from several years ago that distrowatch.com was tracking about 350 different Linux distros. Curious to know what the situation is like now, I went to their home page a few minutes ago, and counted the number of “<option ...>” entries in the “Select Distribution” popup menu at the top, and got 290. This figure also includes a few non-Linux OSes. But even so, I think it’s fair to say the sheer variety of Linux distros has scarcely abated over the last decade.