The number of rounds you should do depends on your experience level and available time. Here's a guide to help you choose.
If you're new to the practice, start with 2-3 rounds. This allows your body to adapt while still providing benefits. Focus on technique rather than duration.
Once comfortable with the basics, 3-4 rounds is the sweet spot for most people. This typically takes 15-20 minutes and provides a full experience.
Experienced practitioners may do 5 or more rounds for deeper states. Wim Hof himself often recommends 3-4 rounds as sufficient for most purposes.