You should replace all ball joints at once if you can afford to do so.
Once one ball joint starts to go bad then it's often soon that the other ball joints will also fail.
However if you can only afford to replace only the bad ball joints right now then that is also okay.
But you should also plan on replacing the other ball joints as well eventually.
Replacing all the ball joints at once also ensures a more consistent and safe suspension system.
Ball joints that are on the same axle also tend o wear out at similar rates, so if one ball joint on the same axle fails then the others are likely not far behind at failing.
Replacing only one ball joint can also affect the vehicle's alignment and make it necessary to get a re alignment after the ball joint replacement.
Replacing all the ball joints at once can save you money in the long run by preventing the need for any future and multiple repairs later on.
You will find that ball joints on each side of the vehicle tend to have similar wear, which means it's common to have them replaced in sets or matching pairs.