You can either go by function
tourer + commuter + off-roader, so 3 is many people's ideal
or my own preference is by engines
a single, a parallel twin, a V-twin
As my old brain can't cope with changing gear sides and patterns they all must be right side changers, 1 up, 3 down, which rules out modern multi's. The V twin's tricky too unless you can afford a Vincent, so I'm on the lookout for an early H-D sportster to complete the collection.