How many national capitals are named after a U.S.president?

Two capitals.

Monrovia (Republic of Liberia) is named in honor of U.S. President James Monroe, a prominent supporter of the colonization of Liberia and the American Colonization Society.

In 1845, Monrovia was the site of the constitutional convention held by the American Colonization Society which drafted the constitution that would two years later be the constitution of an independent and sovereign Republic of Liberia.

Washington, D.C. named in honor of President George Washington, the City of Washington was founded in 1791 to serve as the new national capital. In 1846, Congress returned the land originally ceded by Virginia; in 1871, it created a single municipal government for the remaining portion of the District.