* Dialects are fluid and overlapping: There are no clear-cut boundaries between dialects, and many people speak a blend of features from multiple dialects.
* Data limitations: While there are some linguistic studies on dialect distribution, they are often limited in scope and don't capture the full picture of dialect usage across the entire US population.
* Defining "major" dialect is subjective: There are various classifications of American dialects, and what constitutes a "major" dialect can be debated.
However, based on available information and common classifications, it is likely that General American (also known as Midland American) is spoken by the largest number of people. This dialect is characterized by features found in the Midwest and parts of the Northeast, and it is often considered the "standard" or "unaccented" dialect in American media.
While Southern American English is widely spoken, it is likely to be a close second to General American. Other major dialects, such as New England English or New York City dialect, have smaller geographical reach and therefore fewer speakers.
It's important to remember that these are estimations and generalizations. The actual number of speakers for each dialect is complex and constantly evolving.