I don't know how to word this question but if you can get a general idea of what I'm talking about, that would be great.
Everything has a bottom line, lowest amount, the least you can do. What is the lowest amount of CFM that would be considered adequate or good?
How much CFM you need depends heavily on how much power/heat you need to get rid of and how efficient air flow within the case is, where efficiency would be the percentage of air that picks up meaningful amounts of heat vs how much air goes through without picking much/any heat.
If you have a case with a pair of 120mm front intake and a pair of 140mm fans at the top, the front 140mm will have the most direct path to the 120mm fans and most airflow from the 120mm front fans will go straight up that fan without picking much/any heat since there are almost no significant heat-dissipating components in that path... maybe 15W from HDDs/SSDs and 10W from motherboard components near the inner edge.
I prefer high Cfm fans on a fancontroller, that way you can tweak them to low speed for quiet cooling or crank them up for performance cooling for gaming/clocking sessions
but as Brett said, highest you can get at a noise level you can live with is what you want