I got these in a large German button, buckle and thread grouping, in the original WW2 manufacturer's box. They are 14 mm, zinc, painted a dark field gray. These are shirt-sized, not the big ones for breadbags and ponchos and such.
6 buttons is enough for the standard 4 button German shirt (with two cuff buttons).
If you have the shirt variation with pockets or shoulder board loops, you will need additional buttons. 4 button German shirt with two pockets = 8 buttons; 4 button German shirt with pockets and shoulder board loops = 10 buttons.