They mentioned that in some of their twitch streams relatively short after Unreal 4s initial release. I don't remember the number, but it was smaller than what I thought it would be. Plus of course the people not working for epic who contribute to the source code on github.
Do you at least remember how big you thought it would be? Currently I have no idea of even the magnitude of the team.
I believe it was around 70-80 engineers, and the number mentioned in the twitch stream was lower (keep in mind I have no experience when it comes to writing large 3d engines. maybe my expectation was ridiculous to begin with). I actually tried to hunt down the video in which they mentioned that, but Epic is just too productive when it comes to producing videos, and I gave up. :D
I would say 70 is definitely very high for an engine. Most games don't even have that many programmers overall. Although I guess this also comes down to definitions. Like do the IDE/tool builders also go under the engine team? Then 70 isn't too high.