On a mass spectrum, the carbon 13 isotope peak appears at approximately one mass unit higher (the actual mass delta 1.00335) than the carbon 12 ion peak. The intensity of these isotopes is proportional to the relative abundance of the naturally occurring isotopes. The relative abundance of the two isotopes is 12C ≈ 98.9% and 13C ≈ 1.1%.
Without any structural information, we can estimate a general ballpark figure for the number of carbons using the peak intensities for the 12C and 13C ion peaks.
For the 12C ion peak (m/z 386.4) shown below, the upper limit on the number of carbons is calculated at 386.4 / 12 = 32.2. Rounding down, we arrive at 32 carbons. Based on this information, the intensity of the 13C peak is expected at 32 * 1.1% = 35.2%.
Experimentally, the intensity of the 13C peak is 23.6% with the 12C peak at 100%. The calculation is (23.6 / 100 *100%) / 1.1% = 21.4.
To estimate the # of Carbons ≈ (Int13C/Int12C * 100%) / 1.1%
*Note: Instrument and the type of experiment can influence the intensity of the 13C peak and thus produce a less reliable estimate. Ideally, the result is best evaluated in conjunction with the carbon count from a 13C NMR.