I constantly get numbing sensation in my legs. What causes this?

Neuropathy. This can be a compressed nerve in your spine or if you have diabetes this can cause general damage over time.
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.