@s4w: The link in my previous post shows how.

I know that and use 9 connections for a decade, never had a single problem. But WinSCP does not allow more than that, this is why I wrote this request, How to allow more?

Thanks for your recommendation.

I do not recommend it for stability reasons.
If you want to risk it anyway, see:
https://winscp.net/eng/docs/rawconfig#queuetransferlimitmax

Suggestion is to increase the "Maximum number of transfers at the same time" to at least 32. Though many transfer utilities have default 10 but most of Linux utilities allow up to 128 connections.