Complex sentence: The result is that wave groups (called sets by surfers) can have a large number of waves: from about seven waves per group in the storm, this rises to 20 and more in swells from very distant storms.
Split into simpler sentences: The result is that wave groups (called sets by surfers) can have a large number of waves. From about seven waves per group in the storm, this rises to 20 and more in swells from very distant storms.