I'd estimate about 300 organized veteran devs could do everything Elon is trying to do, but on nostr, over the course of 3-6 years, with consecutive chunks of progress on different components.
A team of more like 1000 (again, assuming they're all cream of the crop) could do it in more like 1-2 years by having more teams building parts concurrently instead of consecutively; however, their work would likely be lower quality than the smaller group taking several years.
And in real life we have a disorganized team that will grow to thousands of people if it hasn't already. Some are the cream of the crop, some are student hobbyists who don't even use their own work. They will make components both concurrently and consecutively, often multiple separate times for the same component, and by the time nostr is an "everything app" it will already be 3 different competing "everything apps" and that's part of the fun. The numbers I'm crunching here are just for thought provoking purposes