How many programmers realistically would it take to make a high end game in UE4?

Yep prolly the sweetest project to come out of the UDK program.

that said it has a serious lack of narrative and is really only multiplayer. I would saying if you are looking to do something with a single player or at least more of a narrative you are looking at closer to 20-30 to do something approaching AAA quality. And that would be everyone being self motivated bad ■■■■■ that are uber productive.