The maximum number of carbon atoms found in naturally occurring monosaccharides is nine (9).

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Explanation:

Monosaccharides are classified by the number of carbon atoms:

No. of Carbons	Type	Example
3	Triose	Glyceraldehyde, Dihydroxyacetone
4	Tetrose	Erythrose
5	Pentose	Ribose, Xylose
6	Hexose	Glucose, Fructose
7	Heptose	Sedoheptulose
8	Octose	2-Keto-3-deoxy-octonate
9	Nonose	Neuraminic acid (and its derivative sialic acid)