In total there are twenty-three synchronize points in the game. There are two in Versailles, which I would recommend getting before progressing in the story so you don't have to come back. There are three in La Bièvre, three in Lle de La Citè, three in Le Louvre, three in Le Marais, three in Ventre de Paris, three in Le Quartier Latin, and the final three are in Les Invalides.
And even if you don't want the achievement they are still good to get so that you may fast travel.
Edit: For those looking for the final Viewpoint in Versailles. Go to your Map, find the Theatre; Right beside that there is a "V" icon;That is Versailles. Click the "V" to travel to Versailles then pull up your map again. you will see the missing viewpoint < credit PapaMarbleMocha