You are correct, ADA defines an “employer” as any person who is 1) engaged in an industry affecting commerce; 2) employes 15 or more full-time employees each work day; and 3) for at least 20 or more calendar weeks in the year. In the context of physical spaces, ADA would not apply to companies with fewer than 15 employees. However, courts don’t seem to have come to a consensus on what digital compliance really should look like. Because websites can be accessed anywhere in the country, small business owners might potentially face lawsuits in unfavorable jurisdictions. If you are a small business owner, I would recommend consulting with an accessibility lawyer and ask what they recommend.
The ADA statute identifies who is a person with a disability, who has obligations under the ADA, general non-discrimination requirements and other basic obligations. It delegates fleshing out those obligations to federal agencies. The agencies issue regulations and design standards. The regulations have the details on the rights of people with disabilities and responsibilities of employers, state and local governments, transportation providers, businesses and non-profit organizations. The design standards specify how many entrances need to be accessible, how many toilet rooms and the design for those elements.  To know what the ADA requires, you need to read the law, regulations and design standards.  
The large number of people who have disabilities, coupled with the challenges that they face, is one of the reasons that the Americans with Disabilities Act (ADA) was passed in 1990.² As its name suggests, the ADA is designed to protect individuals with disabilities in the United States. The ADA essentially makes it illegal for any government entity or business to provide goods and services to the general public without ensuring that the entities are accessible by people with disabilities. In today’s digitally driven world, many businesses fail to follow web accessibility best practices. In fact, this is why the Supplemental Advanced Notice of Proposed Rulemaking (SANPRM) was created by the U.S. Department of Justice (DOJ). To ensure that they are implementing digital accessibility best practices, organizations are encouraged to use the WCAG 2.1 technical requirements.³
The ADA is the Americans with Disabilities Act, passed almost 30 years ago in 1990. This act was originally put in motion pertaining solely to physical location. It requires establishments to provide people with disabilities easy access to various levels throughout the business. This act was set to achieve an equal experience to all people, handicapped or not. 
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