Alternator phases are determined by the number of coils in the stator. Three-phase-alternators produce more consistent power than single-phase alternators. Which one you need should be determined by the load or application.
Your alternators voltage determines
the types of devices or systems it can power.
Common voltage include:
480/277V, 440/254V,
220/127V, 208/120V
Single phase 240/120V 220/110V
Common voltage include:
346/600, 4100/2100.13800/7620
Common voltage include:
3 phase 4160/2402V, 13800/7968V