There have been no conclusive studies conducted that determine how many insects are swallowed by humans. The actual probability of a person swallowing an insect while asleep is low, as most insects view humans as predators and do their best to avoid them.
For a person to actually swallow an insect while sleeping, a number of circumstances must occur at once. The insect would have to be on or around the person's bed. Next, the insect would have to enter a person's mouth. Insects naturally fear predators larger than themselves, so it would be odd for them to enter a human mouth. Lastly, the person who has an insect in his mouth would have to swallow said insect. Somehow, the insect would have to trigger a person's swallowing reflex, which would be incredibly hard to do while the person is sleeping. These circumstances alone would be rare to achieve and make it almost impossible for a person to swallow a bug while sleeping.