How many books are there in the Tarzan series?

There are twenty-five books by Edgar Rice Burroughs featuring Tarzan :

Tarzan of the Apes (1912)

The Return of Tarzan (1913)

The Beasts of Tarzan (1914)

The Son of Tarzan (1914)

Tarzan and the Jewels of Opar (1916)

Jungle Tales of Tarzan, 1917 [1916].

Tarzan the Untamed, 1921 [1919].

Tarzan the Terrible (1921)

Tarzan and the Golden Lion, 1923 [1922].

Tarzan and the Ant Men (1924)

Tarzan, Lord of the Jungle, 1928 [1927].

Tarzan and the Lost Empire (1928)

Tarzan at the Earth's Core (1929)

Tarzan the Invincible (1930-31).

Tarzan Triumphant (1931)

Tarzan and the City of Gold (1932)

Tarzan and the Lion Man, 1934 [1933].

Tarzan and the Leopard Men (1935)

Tarzan's Quest, 1936 [1935].

Tarzan the Magnificent, 1937 [1936].

Tarzan and the Forbidden City (1938)

Tarzan and the Foreign Legion (1947)

Tarzan and the Tarzan Twins (1963, for younger readers)

Tarzan and the Madman (1964)

Tarzan and the Castaways (1965)