Is polygyny the Sunnah of the Prophet? Is it more virtuous to practise polygyny?

Polygyny was practised by the Prophet ﷺ, but most scholars hold that its basic ruling is permissible, not a sunnah recommended generally to all men.
Surah an-Nisa' verse 3 permits up to four, but follows with "if you fear you will not be just, then only one... that is nearer to not doing injustice". On this basis, some Shafi'i scholars state that it is better to be content with one wife for one who has no need. Other scholars see it as encouraged for a man who has the need and the means and is confident he can be just. So whether it is preferable depends on the individual's circumstances, not on a general ruling. See Ruling on polygyny.