I want use ue4 on building model,and the Compoments may be more than 2000000.I want to know the number of actors can UE4 created.
It depends, you can only have a limited number of objects at a time due to draw calls, which depends on your system hardware specs. Something like 1,000 is OK. But, the overall map can have much more, it just loads in when you get close enough.
Some clever engineering would be required to do that kind of object count but in theory it is possible.