I'm 21 year old guy and i have been urinating a lot I usually go 3 to 4 times a day but know i go 8 should i see a doctor?

I'm no doctor but I would say it depends on if this change corresponded with a habit change.

Increased urination can be caused simply by drinking more water in the day, spending the majority of your time in cooler places (like in air-conditioning), or spending less time in hotter or drier areas when you normally sweat. Diet can also be a factor because your body tends to store more water when you eat carbs regularly than when you refrain from eating much carbs for a few days.

If this keeps going on, it's probably safest to consult a doctor since increased urination can also mean that your bladder muscle has weakened for some reason, or it could be a sign of disease, like Diabetes (not that I'm saying you have that because for someone your age it is very unlikely, but I'm just giving an example).