Unless the spiders are being consumed on purpose, the average person is unlikely ever to swallow a spider. Occasionally, a ballooning spider might accidentally drift into a human mouth, but this is such an exceptional event that it would be difficult to derive an average figure for the general population.
The myth of unintentionally swallowed spiders probably began with a chain email sent by an Internet columnist named Lisa Holst in the early 1990s. Holst spread the fabricated figure of four spiders per year as part of an experiment in public susceptibility to unfounded "facts." The myth took off and is routinely quoted by various sources.