You won't find maintenance information in API 6A, as it is a new valve standard only. API 621 is a RP for reconditioning of specific types of valves. You may find the answer you are looking for there. I am not familiar with the contents so, the testing requirements may be more or less stringent than the standard API 6A FAT requirements you listed. Your customer should be able to tell you what they expect to see for a seat test requirement. With that large of a valve quantity I believe your customer would be a major operator who will have internal refurbishment requirements, and may assign a third party inspector to witness/review your testing records and procedures.

Edit for grammar and spelling.

As stated by SPDL310, API 6A does not include maintenance information.

However, API STD 6AR covers "Repair and Remanufacture of Wellhead and Tree Equipment", including hydrotest requirements. Per Section 1 of 6AR, it applies "when specified by the user/purchaser of the equipment", so you may want to check if your customer requires the valves to meet 6AR or another specification. Alternatively, you can hydrotest per API 6AR, which simply refers you back to API 6A except that the hydrostatic body test doesn't have to exceed MAWP if you haven't replaced pressure containing parts.

API 6AR Section 11: "The quality control requirements for factory acceptance testing of assembled equipment shall conform to API 6A for the PSL corresponding to the RSL to which the equipment is being repaired or remanufactured. When pressure containing parts have not been replaced or remanufactured, the hydrostatic body test pressure is only required to be minimum of the rated working pressure."

Note that the current version of 6AR is designated as a standard (STD) and was published in 2019. Prior to 2019, the 6AR requirements were included in 6A for a few editions and before that were a separate recommended practice (RP).

Thank you SPDL310 and jmec87!