I mention this as you may want to validate my premise that sample size is nothing more than what you feel comfortable with.
But remember - you have to be ready to give your reasoning on why you chose the sample and the size you did. Duh or Well, why not? does not fly well with auditors.
Key word - REASONING
Key Question : Please explain why and how you chose the sample you chose and the size.
Sample Size II
If you ask your registrar what sampling plan they use to determine sample size, you will find them hemmmming and hawwwwing at best. In their opening comments to your group during the meeting before the audit starts, as well as during the exit meeting, every registrar I have ever witnessed has spoken about how they take a sample of your system and (to limit their liability) they will say that just because they did not find something that does not mean there were no nonconformities. None has ever cited a valid sampling plan, much less sample size (valid = based on something other than speculation). I guarantee they will NOT cite ANSI/ASQC Z1.4-1993 or the old standby MIL-STD-105.
ISO 19011 - Quality and Environmental Management Systems Auditing Forum Discussions
. 
