One.
I sound like a One Note Sally but my advice is to spend $32 to get a Uni-T light meter and use the attached document to get your grow to its light saturation point.
The drag racing expression is "Ain't no substitute for cubic inches.". When it comes to increasing the yield of a cannabis grow, "Ain't no substitute for hitting the light saturation point". That really rolls off the tongue, doesn't it?
Nutes are not food to a plant. Light is how a plant makes food and growers routinely don't give their plants enough light for the plant to reach its genetic potential. It's tough to see growers spend a lot of time nurturing their plants and then ending up with a yield of X grams when the could well get 2X by turning up the dimmer.
This table is derived from the paper I've cited. For a change in 50µmol, the yield from the strains that were being grown increased by the 5 in the ∆ column. Those look like small percentages but moving from, say, 700µmol to just 800µmol, yielded almost a 10% increase.