Why limit it to the arbitrary number? Just allow the user to select which boats to show data (and the type of data) on. It would allow for more customization and at different zoom settings, it can naturally make more (or less) space for the displayed data.
As for the type of data, it would be nice to see numerical numbers for course, SOG, and VMG. It could be helpful to have the option to see all or some of the above data for each boat, instead of only SOG.