OK, Chuck, here's what I get. I've got the 30 December 2013 copy of the FAA Registration database:

27,964 aircraft in the registry licensed as Experimental Amateur-Built.

5,780 additional aircraft have homebuilt-like names ("RV-8", "Lancair 320") but their airworthiness entries were blank.

10,671 aircraft that are in the Deregistered aircraft list were licensed as Experimental Amateur-Built. I'm estimating that another 4,000-5,000 deregistered homebuilts had blank airworthiness entries.

So: My estimate is that roughly 48,000 homebuilts have been licensed in the US since the Experimental Amateur-Built category was created in the early '50s. Probably could round that up to an even 50K.

Ron Wanttaja