500 Scientists
The rough estimate of resources required to develop - for mice - the medical capabilities called for by the Strategies for Engineered Negligible Senescence (SENS) is presently $1 billion over ten years, give or take.
Each of these [six lines of research] would require total funding in the range of $2m to $15m per year, spread over at least three and sometimes ~15 research teams. These teams will typically be working in a university or other research setting. [The lines of research] span six of the seven types of "damage" that Dr. de Grey has identified as the key intermediates in aging; the one not listed here is cell loss, whose rectification by stem cell and growth factor therapies is the subject of sufficient existing work worldwide.
That's for the whole spectrum of longevity therapies: engineering the body to make cancer impossible; replacing lost cells; ensuring mitochondrial DNA damage can no longer cause issues; destroying unwanted cells that cause damage; breaking down crosslinks and amyloid that gum up biochemistry; removing hard-to-degrade biochemicals in old cells. Given all that, you should be able to rejuvenate aged mice, and extend their healthy lives considerably. Then it's onto moving the technology to work for humans, where the cost really starts to rack up - but with the technology demonstrated in mice, there should be plenty of enthusiasm to pay that cost.
What does a billion dollars and ten years really look like when you're taking about warm bodies, concrete and conferences? It turns out to represent something like 500 researchers, plus resources for equipment, facilities and support staff, if you keep things lean and distributed, making the best use of existing research facilities and ongoing programs.
If you apply the 1:9:90 rule to a research community, you can expect that a 500-scientist strong group will include perhaps 5 researchers who are very respected and appear in the media in connection with their research, 50 who are well known in the field and very capable, and the remaining 445 ranging from research associates to skilled scientists yet to reach the heights of their careers. This community might take the form of ten dedicated laboratories at large universities, a few for-profit enterprises, and more than fifty significant initiatives within other large research organizations.
For comparison, that is considerably larger than the present calorie restriction research community but considerably smaller than either the cancer or Alzheimer's research community. Calorie restriction research and development is probably well over $1 billion in investment to date, but only if you count funds for trials and commercialization employed by companies like Sirtris; I would imagine that basic and animal research has consumed rather less than that.
At the present time, I would be surprised to find more than 50 scientists worldwide working to develop biotechnologies that would fit into SENS as-is. Outside the regenerative medicine and cell therapy community, that is. Clearly, there is a way to go for fundraising and other efforts to influence the direction of research in the broader scientific community.