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.