50 cercles non joints, comment procéder sans les faire un par un ?

Are the circles touching? If they’re not touching, select them and apply a Pathfinder Unite. Pathfinder will close the circles but it will group them all...so, if stacking order is important or if the circles are in different layers this might not work for you.

if circles are in different layers, select them all and apply an Add Pathfinder Effect, then go to Object->Expand Appearance. This method will also created a group for each circle with two items in it, one for the fill and one for the stroke. You could remove either stroke or fill to avoid having two items in the group, then you could re apply the fill or stroke, whichever is easier.

You can use a script, this for instance: https://www.samolevsky.com/blogs/adobe-illustrator-scripts/close-open-paths?srsltid=AU7gw4XcKDWdSCrUCF5n_COWwAqZzLxzTjwEGwl3dg_Ph6eLh3wFfr7Z