tohbin,

Thanks for reposting with more information. Unfortunately I am still confuse because of the terms you are using, test and inspection. Is this an inspection after a test to determine if the test is operating correctly or is this simply the number of units to test to assure the process average is 200 - 500 ppm?

If you want to determine how many units to sample inspect to assure the ppm is 200 to 500, you need to know what risk level you are willing to accept that you might make a bad decision based on the sample results. In other words, if you have a million parts and there are 200 defectives in that universe (200 ppm) how many do you have to look at to be sure there are no more than 200 ppm. You would have to look at many more to be a 99% sure than you would have to look at to be 50% sure. Another complication is the sample size to discriminate between 200 ppm and 100,000 ppm will be much smaller than the sample size to discriminate between 200 ppm and 1,000 ppm.

As Mike and Dave indicated, this is not a simple question, it involves determining both supplier and customer risk factors. My suggestion is you need to learn more about the basics of sampling. There are a number of good texts available. Ed Schilling has an excellent advanced text that is available through ASQ. Dale besterfield's book, Quality Control, has good sound basic information on setting up a sampling scheme.