Dowload speed

I have version 5.15.9 of WinSCP. I have experienced download times up to 50-60 Mb/s on different machines. That means that one file may be downloaded on ~50Mb/s, while if two files there would be ~25Mb/s for each. At the moment I am able to download several files simultaneously, lets say 4 files each of 12-13 Mb/s. But if I do just one file, it is still 12-13 Mb/s. I cant find the setting how to download ONE file with ~50Mb/s - it should be possible?

There's no magical setting for that. There are some settings that may have an effect, but it's typically unlikely. A transfer speed is limited by other factors, not only by the bandwidth.
What protocol are you using?
See also https://winscp.net/eng/docs/faq_slow