How to validate/verify benzoic acid percentage
Depends on your product matrix but if at low quantities, I'd suggest HPLC for accurate measurement which could be part of validating any ingredient addition.
Monitoring, I'd use ingredient addition records which can be anything from the prone to inaccuracy of manual records to highly accurate if systems are well designed and automated (although some companies do use HPLC for routine chemical testing before release to next stage depending on what they're making).
Can you tell us more about your product as that will have an impact?
Using an external lab for HPLC is definitely a possibility for validation.
We have a system that records ingredient use for production but it is not very specific for each batch and we could make many batches in one production. I am unsure if that is acceptable.
Ok if getting an internal HPLC isn't an option, certainly external for validation.
Is the benzoic acid the only acidulant? If so a titration / total acidity test is an option but the accuracy might not be great if it's in a low quantity. I'd definitely tighten up your records batch to batch if you can as that's really the monitoring of your control (i.e. how much you're adding).