We are (recently) Tier 1 to GM. We were Tier 1 in the past when Delphi was still "under the umbrella". We're very used to having to perform to "GP-12" requirements.
We have been Tier 2 and Tier 3 for many years, with most of our Tier 1 customers supplying GM, and have always had the "GP-12" requirement. It is not that big a deal when it is planned for and has proven to "verify" the Production Control Plan.
I believe (without looking it up) the default number of pieces is 1200, unless otherwise specified. It has only been recently that we have had customers "specify" a quantity and then it is mostly a length of time or number of shipments, as opposed to a specific quantity of parts.
Bill