I would not recommend testing samples of all batches, but you should have a rolling microbiological audit plan in place covering.
Settle plates for TVC, Yeasts and moulds
Swabs of product, work surfaces and peoples hands for listeria, salmonella, enteros and e-coli.
Your competent contractor should be able to put alert/action limits to the range of testing and provide a report of the conclusions in layman’s terms for you... I would add an extra part to the report for corrective action taken, date and sign off etc. The results will validate you GMP procedures or will point you to where you need more attention. Customers like to see this even though BRC does not specify micro testing. FDA IMS approval does and monthly product samples need to be tested. I think this is over the top and have always operated the audit on a quarterly basis.
In addition to the micro testing of course there is other testing as mentioned by other members e.g.:
Food contact approval - migration and heavy metal testing - responsibility of supplier of raw material
Ink migration – testing for ink migration through printed packaging and transfer from front to back in winding / stacking – responsibility of converter
Hope this helps.
Regards,
Simon