How Many Developers?
One handful works on the core of the system in their free time, and another handful helps with Debian GNU/Hurd and Arch Hurd packaging. Also, an additional handful of former developers are still available for answering technical questions, but are not participating in the current development anymore.
In the past (that is, a lot of years ago), the FSF did pay a few developers for working full time on the GNU Hurd. But that was for a limited amount of time only, and evidently, it was too little for getting the system into a competitive state. Nowadays, it's only unpaid (apart from some bounties) and free-time volunteers' work.
In contrast to the Linux kernel, there is no industry involvement in development. For one, this is a good thing: independency; no conflicts of interests. For another, it is also a bad thing: no dedicated full-time manpower -- which matters a lot.
Why So Few?
We can only speculate. One major problem might be that the architectural benefits are generally perceived as very abstract, with little practical benefit. We currently don't have many tools that are actually making use of all the possibilities.
Another reason is that it's been taking too long. Today, most people don't believe it will ever be ready for production use, and thus would consider involvement a waste of time. This latter point is invalid, of course, as learning can never be a waste of time. The same holds for the challenges raised by the GNU Hurd -- we can only learn and improve upon working on them.
For likely the same reasons there is no industry interest in the GNU Hurd: its advantages are too abstract and incomplete for being of interest there.
As for the scientific sector, the GNU Hurd projects was rather about using a microkernel intead of doing research on them, for example. But, there have been some projects and theses done, and some scientific papers published on GNU Hurd topics, and we're generally very interested in further such projects.
Attracting New Faces
We're an open project: any interested party (you!) are very welcome to start contributing. Mentoring is possible, too, to help you get started.
Likewise, for reaching out to new developers, we're participating in Google's Summer of Code program.