Each individual font weight or style that you have selected is counted as one font sync. For example, selecting the regular, italic, and bold fonts from the Proxima Nova family is three fonts.
The fonts that you currently have synced are listed on your Synced fonts page. You can remove any font families that you no longer want to sync by clicking the “Remove” link next to the font name; see the “managing your synced fonts” help page for more information.
If you are at your sync limit and would like to use additional fonts, you may upgrade to a higher-level plan.