I want to start taking B12 vitiamin for energy is 500 mg enough 1 tab a day?

Maybe. Unless you are known to be vitamin B12 deficient, there is much debate about whether or not you'll get any energy benefit from supplementing b12: there is no good data to support this, however there are many anecdotal stories from those who feel better. That said, 500 micrograms per day should be safe and hopefully help you feel better and more energized!
Too much. If you do not have a vitamin deficiency, taking vitamins can be detrimental, even increase your cancer risk. A placebo ("sugar pill") is known to boost energy levels in some.