How many words should my two year old speak?

At least 20. The average is 40-50 or so, but many kids its about 20+ and beginning to string two words together. However, as important is comprehension of speech. So much so that newer studies suggest kids who speak a little slower but understand and follow directions well, likely will catch up just fine by 5-6 years old. There is a spectrum of normal. If slower, check with you pediatrician.
50+ words. A 2 year old should have greater than a 50 word vocabulary.