Many sperm may reach an egg in a Fallopian tube at the same time. What prevents more than one sperm from fertilizing an egg?
It's due to the fast and slow block response to prevent polyspermy; which is fertilization of the egg with multiple sperm.
One of my favorite topics! Briefly, the egg's membrane potential changes once the sperm reached it causing the vitelline layer of the egg to elevate (lift off) due to the cortical granule reaction. The zona pellucida then hardens preventing any other sperm from entering the egg.