Yr question as far as "representative" is involved is primarily statistical. Detection is a different aspect
And statistically similar to sampling for Salmonella if you are referring to an isolated lot of individual packages of product derived from (possibly) various sources.
The typical statistical requirement is one of determining a sample size such that one can state that there is, for example, a 95% probability (ie confidence level) that the level of contamination in the lot is less than X%. Numerous publications detail the solution for this, eg the classic publication Microorganisms in Food Vol.1(?) from ICMSF (later updated in vol8 (?) which latter may be available on-line as google book).
In practice, some interpretations arbitrarily set a standard such that compliance (eg null detection result for 15 random samples) allows one to "claim" that the lot is "free" of Salmonella. Strictly speaking, this is of limited statistical meaning of course.
Related extracts from some publications may also exist on this forum if you do a little searching.
Rgds / Charles.C