The human brain can automatically quantify objects and events. The mismatch negativity (MMN) of the event-related potential (ERP) was recently found to reflect one such ability in the auditory modality. The present study aimed to further validate the finding. ERPs were recorded in adult humans who passively listened to a series of 300-ms sequences of tones. The sequences occurred at 300-ms silent intervals. They comprised a total of six tones that each was either 440 or 660 Hz in frequency. MMN was found for a rare 4:2 ratio interspersed with a frequent 3:3 ratio between the tones of the different frequencies in a sequence, suggesting the ability of the human brain to preattentively discriminate numbers of sounds of specific frequencies at least at a 3:2 ratio.