Well, I choose the one in which Tichy flies into some time-warping zones and subsequently meets the copies of himself ... I don't remember the number though :]
21? It's my fav too
21st is also about time travels
BUT
In
7th Tichy has some problems with his rocket, flies into 'time whirpools' or however it is called in english and meets many copies of himself.
In
21st there is nothing about time travels. Tichy lands on a planet and learns much about their culture and religion.
So, I think that
Draco Volantus was talking about
20th. Tichy meets there only 1 copy of himself, travels forwards in time and is involved in repairing past... that leads to a total disaster.
(Of course, i wrote that after looking into my book for a short while.)