It depends on the application. When you have a cooling fan bank of 20 fans, they turn on/off each individual fan.
Smaller banks, say 2~4 may have one VFD, switchable between more than one fan.
Motors can have a much lower efficiency outside their rated design point. It is be more cost effective in some cases to break up the service into multiple pumps. This also helps in a N+1 type redundancy that allows individual motor/service to be taken out of service while doing maintenance and keeping production up.
Most refinery service is designed for a specific flow rate. No only the pumps, but the separators, piping, reactors, etc all run at a set, non-varying rate for many services. There is not a need to vary, but a need to design the proper size head vs flow in the impeller and it runs there for a decade.
With a VFD you can do the same job with a smaller motor simply because it doesn't have to be sized for startup loads.
Motors can have a much lower efficiency outside their rated design point.
Precisely my point. The power factor losses for which a capacitor is meant to compensate are only an estimate of load. A VFD can adjust for same much more accurately in real time.
BTW, with costs for electricity as high as they are in California, we have much more incentive to invest in even small power savings than in Texas for example.