Countless places in the USA are named after George Washington. How many were named after his wife?

One, and that would be Dandridge, Tennessee

"Historic Dandridge bears the distinction of being the only town in the United States name after Martha Dandridge Washington, wife of our first President, George Washington."

https://www.dandridgetn.gov/plan-your-visit/history/

Fun fact about Dandridge, TN: During the construction of the Douglas Dam on the French Broad River in 1942, downtown Dandrige was planned to be flooded by the reservoir behind the dam. Residents of the town successfully petitioned then First Lady Eleanor Roosevelt, pointing out that Dandridge was the only town in the United States named for the first First Lady. Mrs. Roosevelt ensured that a levee was built between downtown Dandridge and the reservoir to protect the town from flooding.

https://en.wikipedia.org/wiki/Douglas_Dam