After compiling c++ project i have in binaries folder two files UE4Editor-Project.dll and UE4Editor-Project-9982.dll. Why need second file? And how to compile into single file?

2nd one is for hot reload, UBT can’t delete or modify old file when it’s operational, so instead of it it create new one and UE4 editor load it swaping the old one in memory. The hot reload file can be deleted after you turn off UE4 editor, in fact UBT gonna do that for you want you next time you build without editor open, as well when you package you will have only one dll. So you don’t really need to care about it.