I had a terrible case of numbness after my first century - it was in my right (dominant) hand, between the thumb and index/middle fingers (so apparently the median nerve). This was quite hilarious because I was teaching college courses, and since I could not grip anything properly I had to hold the chalk in my fist to write anything on the board.
I was able to grip things well enough for daily life (writing, starting a car, etc.) after about two weeks, and full mobility returned after about three weeks.
Part of the culprit was certainly bike fit. I was also a novice distance rider with a bit of the death grip later in the ride, and in addition, had reversed the brakes - so the front brake was operated by my right hand. I worked on bike fit and core strength, which made multiple hand positions easier, and also started wearing gloves. I no longer have issues with numbness.