@vishalrao I use Utorrent. So under preferences--bandwidth--Number of Connections, I have changed "Global maximum number of connections" from 200 to 500. I hope, this is what you meant by "control numbers of connections per torrent."
Now trying Default upload/download and trying to download ubuntu 21.04. Will revert back.
One thing I observed while downloading previous was that, my "peers" was always zero. I didn't understand why this was happening. One torrent, I can understand, 4-5 torrents, something's amiss.
@Rickysin I have increased the number of connections and peers to 700 and 80 respectively in Utorrent and now get speeds upto 13-14 MBps on single torrent. Still somewhat better than what I was getting earlier.
Forgot to update this thread.. also need to try with qbittorrent.