If I understand your question correctly, you are having a problem on the amount of testing that you need to conduct to proof the Vit B12 content claimed on your label.
If that is the case, you can look at 21 CFR 111.80 that provides you with guidance on how much to collect for representative samples. See below:
© You need to collect representative samples of a subset of finished batches of each dietary supplement that you manufacture, which you identify through a sound statistical sampling plan (or otherwise every finished batch), before releasing for distribution to verify that the finished batch of dietary supplement meets product specifications established in accordance with § 111.70(e), and as applicable, § 111.70(a)
Also, you would want to ensure that you have a solid Supplier Qualification Program and that the COAs provided by the suppliers are accurate and meeting your specifications by verifying those results.
For a Sound statistical sampling plan most companies use the formula √n +1.
As a corrective Action: You might want to develope SOPs for how to collect representative samples, and that how to Release Product, mentioning here the need of conducting testing of the samples vs specifications and review of the results by your QA team.
Optimally, you'd have batch samples included with each lot so that you could test the batch rather than individual SKUs.
If deviations are found, you will need to update your formulation and labels to match with test results.
I hope this helps!
Edited by Marta2013, 04 June 2023 - 08:51 PM.