In C++ you got this:
https://docs.unrealengine.com/latest/INT/API/Runtime/Core/GenericPlatform/FDisplayMetrics/index.html
First you call this to get the metrics:
FDisplayMetrics Display;
FDisplayMetrics::GetDisplayMetrics(Display);
And then you check how many moniters there is by checking MonitorInfo array size with Display.MonitorInfo.Num()