There are a total of 40 sync point collectibles in Assassin's Creed Unity. They can only be found during Co-Op missions and will give you an upgrade point each. These points can be used to buy new skills. They are also needed for the "I Got Skills" trophy or achievement.
Each co-op mission also gives you 2-3 skill points just for completing it. The rest of the skills can be obtained from story missions. Only after completing all missions and sync points you can fully upgrade everything!
You can see how many sync points you have found by going to the "Progress Tracker" in the pause menu. Then select "Co-Op Missions" and it will show how many there are in every mission and how many you have found.
The starting points of all co-op missions are marked on the map. You can play them solo (private match) or with other players in a public match.