4, 5 or 6 speed or more.
In a performance or race car, it all depends on the power band or torque curve of the engine.
In an ideal world one would change up when the engine reaches its peak power rpm. But again in an ideal world, one would then want the revs to drop to somewhere around the peak torque rpm.
If we have a very narrow power band or peaky engine then we need much closer ratios to stay in the power band and that might not be possible with a 4 or 5 speed box so we go up to 6 speeds or more with something like a DSG style gearbox.
Also the heavier the vehicle (think trucks etc) the more important it becomes to stay around the peak torque rpm.
For a light sports car like a Morgan, 5 well chosen ratios is more than enough.
