Generally two or three are left, for different purposes. Two side by side for bifurcation in the process of ramification. One side bud and central bud when elongating apical sacrifice and wanting a fall back in case. Three buds when desiring a lower primary branch, new apical leader and a sacrifice branch in the same area for development purposes. This last one requires careful wiring and pruning as times proceeds so as to not create knobs or inverse taper at the site.
Like most things in Bonsai one cannot apply a standard rule in every circumstance. Just as there will be exceptions or objections to what i have stated above. Developing Bonsai requires interpretation and application of techniques to suit ones objectives. Knowing how the tree responds to the application and what additional steps will be required when one applies the technique. Hope my rambling makes sense