The answer is: it depends. I remember the lesson my Stats teacher: How many Martians do you have to measure to determine the average height of a Martian? What

The answer is: it depends. I remember the lesson my Stats teacher: How many Martians do you have to measure to determine the average height of a Martian? What if I told you that all Martians are exactly 6 feet tall? The correct way to sample to is to determine the variance in the population. If there is no variance, sample size needs to be only one. If not, there are statistical charts that tell you what your sample size needs to be to have some specific confidence that some percentage of the time your sample will be representative. If your population has a Normal distribution, you can use a Z chart to calculate sample size. Believe it or not, for normal distributions and command variances, a population below 200 is all that is necessary.