What percentage of people are introverted?

Introversion is a continuum, so the share depends on where you draw the line. On Big Five extraversion scales, scores are roughly bell-shaped with most people near the middle; about one in six score more than one standard deviation below the mean, a reasonable definition of strongly introverted. The popular 'half of people are introverts' line comes from Myers-Briggs type data, which forces a binary split.

Source: John & Srivastava (1999), Big Five Inventory norms; Myers-Briggs Company type tables