There is not a threshold number of employees for the ADA accessibility rules for public facilities to apply. Here are some resources you may find helpful:
Several ADA FAQs regarding State and Local Governments
Q. Does title II cover a public entity's employment policies and practices?
A. Yes. Title II prohibits all public entities, regardless of the size of their work force, from discriminating in employment against qualified individuals with disabilities. In addition to title II's employment coverage, title I of the ADA and section 504 of the Rehabilitation Act of 1973 prohibit employment discrimination against qualified individuals with disabilities by certain public entities.
Q. What changes must a public entity make to its existing facilities to make them accessible?
A. A public entity must ensure that individuals with disabilities are not excluded from services, programs, and activities because existing buildings are inaccessible. A State or local government's programs, when viewed in their entirety, must be readily accessible to and usable by individuals with disabilities. This standard, known as "program accessibility," applies to facilities of a public entity that existed on January 26, 1992. Public entities do not necessarily have to make each of their existing facilities accessible. They may provide program accessibility by a number of methods including alteration of existing facilities, acquisition or construction of additional facilities, relocation of a service or program to an accessible facility, or provision of services at alternate accessible sites.
Q. What does title II require for new construction and alterations?
A. The ADA requires that all new buildings constructed by a State or local government be accessible. In addition, when a State or local government undertakes alterations to a building, it must make the altered portions accessible.
Q. How will a state or local government know that a new building is accessible?
A. A state or local government will be in compliance with the ADA for new construction and alterations if it follows either of two accessibility standards. It can choose either the Uniform Federal Accessibility Standards or the Americans with Disabilities Act Accessibility Guidelines for Buildings and Facilities, which is the standard that must be used for public accommodations and commercial facilities under title III of the ADA. If the state or local government chooses the ADA Accessibility Guidelines, it is not entitled to the elevator exemption (which permits certain private buildings under three stories or under 3,000 square feet per floor to be constructed without an elevator).
Here is a link to MRSC’s Americans with Disabilities Act page.
For more specific questions, there is an ADA Hotline number staffed by Department of Justice accessibility experts (800-514-0301). The ADA has a website with Information and Technical Assistance at www.ada.gov.