The approach would depend on your resources and also the regulatory requirement of the country where the product is to be marketed. You could develop a chemical marker for each herb individually for which there is an available reference standard, and then determine if the marker from each herb can be recovered in the mixture. Alternatively, you can fingerprint each herb using LC or GC with varying detector types, and then recover the sum of the fingerprints from your mixture using standard software and compare to your expected result to determine percent recovery and method suitability. Alternatively, you could opt for a combination of methods - e.g. a specific analytical method for a single compound for some herbs, and a non-specific test for others where there is no suitable method/reference standard available (e.g. Folin-polyphenols) and determine if you can recover expected results from their mixture. Typically, there is no set regulatory requirement, aside from that whatever is done to standardize an herbal product must be supported by scientifically valid and repeatable methodology.

GC-MS analysis is more suitable for quatitative optimization for herbal products...development.

Although GC-MS is a good method but it depends weather you got the facility. The next option could be any chromatographic approach which is available.

HPTLC could be of help and it is cheaper.