What is the number of samples taken from the S.G 1 layer, As a measure to ensure the quality backfill layer ?

Samples are usually taken from the stock pile for uqality testing before it is used as backfill for SG. In case if we find doubt in the result of testing or difficulties in  executing the preparartion of SG samples can be collected from the site and send to Lab for further verification of the quality of the soil but there are no specified frequency for that.For collecting soil samples form stock pile there are specified frequency.