Quoting Richard Price: Andrew,
If I recall correctly, the therm is something like "one spring per throttle spindle".
The standard throttle body return spring has been OK with scrutineers all the time I've been running my car.
MSA Blue Book J 5.4.2. Be equipped with a positive method of throttle closing by meams of external spring/springs so that in the event of failure of any part of the throttle linkage the throttle(s) are sprung closed.
Lee