How to download only one torrent at a time? and other torrents will be in queue.

Go to Settings > Transfers > General, set 1 Download slot and remove mark on Recycle unused upload slots for downloads.

I followed your method, but i still can't download one torrent at a time. Any suggestion?

Correction. When Tixati finished one of the two active downloads it started downloading one torrent at a time. Thanks for the assistence.

Options > Preferences > Queuing > Maximum number of active downloads > 1

This is not any tixati setting that I know of.

In tixati, it is Settings > Transfers > General > Queuing Download slots = 1
without recyling any slots