SwedenS:
Is my point of view that despite of there is no place where a written rule is given related this topic, is not covenient having only ONE internal Auditor, because..Who will audit s/he ?.
If I were you probably would apply the rule of having from 3 to 5 folks for a micro/small company, less than 50 souls and 5 to 10 folks for a medium sized company,let's say less than 200. For bigger plants it is really a matter of making numbers.
I have 2 more points of view :
1) If you have a very limited number of auditors (supose 2 or 3) you have the risk that they would have another urgent duties related their own activities during the week of the audit,hence the importance of having enough people (not to many of course)in order to face that kind of emergencies.
2) I am convinced that in the case of companies that have the problem that the majority of people is not awere or convinced to change their attitude towards quality or any other type of possitive change they consider "allien or strange", the internal audits would have a very helpful "double role" function: One >> Being a very professional auditor during the audit and, of course, avoiding the role of consultant during the audit, but afterwords s/he will be a "change promoter" in his own department in order to push the closing of non conformances, taking care of going deep to find the root causes of problems and the application of preventive actions, convinced that is better to "wash the dirty cloth at home" before external visitors arrive. This second "personality" has been in my experience very very helpful in small/medium sized companies, making possible the establishment of a "quality gang" in most of the cases more effective than an "official" Quality Comitee, that also in most of the cases is only a corporative "show". The activities of quality gang after the internal audits do not brake any "judge and jury" or any other rule.
Qualiman