Sampling size 問題

There is no definite answer for it.

Pr {Z > [(x - mean) * sqrt (sample size) / standard deviation]} > 95%

From the above equation, mean and standard deviation could be determine based on your specification of the product, or use the sample mean and sample standard deviation

The remaining item is the sample size and the sample mean x

If you have bigger sample size, you could have a lower sample mean (or other criteria) to achieve 95% confidence level

If you have smaller sample size, you could have a higher sample mean (or other criteria) to achieve 95% confidence level

This also have physical meaning. With a higher sample size, you would be much confident that the sample properties could represent the population (100,000) properties.