I don't know your product or process, and you aren;t providing much detail, so I can't give much guidance. But if you are using SPC to monitor your process and can create rational subgroups maybe a subgroup sample size appropriate for a X-bar and R chart like 5-10 is reasonable.
There are a lotta books and articles out there on sampling, read-up and find a plan that works for you based on costs and risks.
There are a lotta books and articles out there on sampling, read-up and find a plan that works for you based on costs and risks.