After injuring my wrist my thumb started to get thumb. It has been numb for now 3 1/2 weeks. Could there be nerve damage and how severe if there is?

Thumb numbness. Not sure exactly how you hurt your wrist but you may have injured a part of your median nerve if thumb is numb on the palm side or part of the radial nerve if numb on the top side. Recovery depends on the extent of the injury...Weeks to months.
Numbness. Numbness or loss of sensation after an injury is likely due to a nerve injury, a consultation with a neurologist would be recommended here.