Finding the mM concentration of capric acid (decanoic acid) in coconut oil
Thank you very much for a well-specified question.  If 8% of coconut oil were free capric acid, then as noted in the comments the units of "%", i.e. w/w, w/v, or v/v, would affect the calculation somewhat, but since capric acid likely has a density that is very close to that of bulk coconut oil, the difference will be slight.  8% w/w, and assuming a density of 0.8 g/mL for coconut oil would yield $0.08~{\frac{\rm g~capric~acid}{\rm g~oil}}\times \frac{\rm mol~capric~acid}{172~\rm g~capric~acid}\times0.8\frac{\rm g~oil}{\rm mL}\times\frac{1000~\rm mL}{\rm L}=0.372\frac{\rm mol~capric~acid}{\rm L}=372~\rm{mM}$.

However, I suspect that the capric acid in coconut oil is not free acid.  It is part of triglycerides.  Instead of containing capric acid, the oil contains an ester of capric acid with glycerol.  Triglycerides are molecules where all of the available 3 hydroxyl groups of glycerol are esterified with a fatty acid.  Because triglycerides are large complex molecules, they were historically difficult for chemists to analyze directly.  The method of choice for analysis of plant oils was transesterification or ester hydrolysis, converting the triester triglyceride molecules into three smaller monoester molecules that were easier to detect by analysis.  The 8% figure you are using is probably the result of such an analysis. 

The bottom line is that coconut oil that is "8% capric acid" actually contains very little capric acid, but contains a large amount of glycerol-esterified capric acid.  But only free capric acid is likely to be antimicrobial.  So the capric acid content of coconut oil is unlikely to make it antimicrobial.

The presence of free fatty acids in plant oils is usually a sign of degradation.  In olive oil, an acidity above 0.5% (i.e. of all the fatty acid chains in the olive oil, 1 in 200 are in the free form and 199 in 200 are in the esterified form) is a sign of lower quality.  I would imagine the for coconut oil the situation is similar, although the specific numbers used as quality thresholds may be different.