Linux Weekly News did an excellent job (as usual) with statistics over the recent release of Linux Kernel.
The end result of all this is that a number of the widely-expressed opinions about kernel development turn out to be true. There really are thousands of developers – at least, almost 2,000 who put in at least one patch over the course of the last year. Linus Torvalds is directly responsible for a very small portion of the code which makes it into the kernel. Contemporary kernel development is spread out among a broad group of people, most of whom are paid for the work they do. Overall, the picture is of a broad-based and well-supported development community.