With careful design, you can make practically anything with one extruder. Without careful design, two extruders is best; one for print material and one for support.

To mix flesible features with rigid ones, having a third extruder is needed (preferably direct mount for flexible filaments).

Beyond that, there's the hypothetical conductive filament... There's better progress with this in paste extruion than plastic right now, but you never know.

Soooo, the most for a practical setup is two bowden, one direct mount and one conductive paste extruder... A setup that would look like Frankenstein's monster.