I want IDM to download only one (or only set amount of) file(s) at a time. How can I configure it this way?

IDM has such option, but it works only with Scheduler. You need to add all downloads you need into IDM queue (press "Download Later" in "Download File Info" dialog).

Then open "Scheduler" IDM dialog, select "Files in the Queue" tab (arrow 1 on the image) and set "Download # files at the same time" (arrow 2 on the image).

If you have several queues you need to repeat this for each queue that you have in "Queue" list (arrow 3 on the image). Then you should press "Start queue" toolbar button in the main IDM dialog.

IDM will download all files in the queue one by one but using only # of downloads at the same time. Please note that this option does not work for downloads that you start manually.