What causes numbness in most people's hands?

Carpal tunnel syndr. Is the most frequent cause, a pinched nerve over the medial wrist at the transverse carpal ligament. Sometimes associated with keyboard activities, and maybe an initial sign of diabetic neuropathy.
Many possibilities. Numbness can occur from various reasons. Some potential causes include vitamin deficiency, nerve damage, nerve compression, alcohol over consumption, side effects of different medications, or as a result of different systemic medical conditions such as diabetes. Get evaluated by a professional to determine the diagnosis. Treatment is based on the diagnosis.