Analysis
NAICS 921 is the 2022 subsector for executive, legislative, and other general government support, part of 92 (Public Administration).
It has only one more detailed code, 9211, which covers exactly the same activities; the definition, index entries and statistics are on that page.
County Business Patterns excludes public administration, so there is no establishment count for it.
Official definition
The Executive, Legislative, and Other General Government Support subsector groups offices of government executives, legislative bodies, public finance, and general government support.
Source: US Census Bureau, NAICS 2022 Manual.