There is not one single study done to date that has quantified the number of spiders you eat in your sleep. The common urban legend states that up to 8 spiders are swallowed in your sleep over your lifespan.
In order for you to swallow even one spider, a series of coincidences need to occur in sequence, all of which are unlikely. Even while sleeping, a spider tickle is felt on your face and unconsciously brushed aside. If a spider, suspended on a silk thread, landed in your mouth without touching your lips, your tongue alerts you to the foreign object. Only if that spider hit the exact spot in the back of throat, you might reflexively swallow it. But, the truth is, no spider is going to voluntarily venture into the mouth of a large, warm-blooded predator.