Why does my download manager get only one file at a time?

Downloads from My Drive by link — in your browser or a download manager — can have 8 transfers open at once per account, and every open connection counts. Download managers split each file into several connections: Internet Download Manager opens 8 per file by default, so a single file already uses all 8 and the next one is refused with "Too many file transfers from this account at once (limit 8)". Only the first 4 transfers run at full speed, so more connections don't make a file faster.

Fix: set your download manager to 1 or 2 connections per file — in IDM, Options → Connection → Default max. conn. number — and 4 to 8 files will download side by side. A cancelled download frees its transfers within a few seconds.

This limit is separate from your plan's downloads at once, which counts torrents downloading on your seedbox. To bring down a whole folder, Download folder (above) fetches one file at a time, so it takes only one transfer.