How many vents is too many?
I think I'm unable to install soffit vents in my house (the eaves are only about 6" from the ext. wall). Would there be any point in supplementing ridge and gable vents with an attic fan? The fan wouldn't be pulling cooler air through the soffits, it would be pulling air in through the ridge and gables. Or is any ventilation good ventilation? This is primarily for A/C efficiency in summer, there's currently no moisture problems in the attic.
A fan would help, but you only need 2"-3" wide soffits to install soffit vents. You can use 2" wide continuous strip vents or 2"-3" diameter vents that you can drill holes for.
While a fan will help it will not provide continuous air flow from the soffit line up to the ridge without soffit vents.