What is the minimum number of carbon atoms a ketone may contain

Three. A ketone is a carbonyl compound, meaning there is a carbon oxygen double bond. A ketone is different from an aldehyde as an aldehyde has this C=O at the end of the molecule, while a ketone is on the inside. Thus, there must be a non-terminal carbon for there to be a ketone, and so three carbons are required (minimum), with the C=O at carbon 2.