CMAKE_UNITY_BUILD_BATCH_SIZE
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Cmake/docs/3.21/variable/cmake unity build batch size
CMAKE_UNITY_BUILD_BATCH_SIZE
New in version 3.16.
This variable is used to initialize the UNITY_BUILD_BATCH_SIZE property of targets when they are created. It specifies the default upper limit on the number of source files that may be combined in any one unity source file when unity builds are enabled for a target.
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https://cmake.org/cmake/help/v3.21/variable/CMAKE_UNITY_BUILD_BATCH_SIZE.html