There are various editions of "Tales of the Cthulhu Mythos", many without a volume number.
Far as I can tell, any marked as Volume 1 will have been published between 1971 and 1975 and feature exactly ten stories proper (leaving aside essays, forewords and the like) by six authors: The Call of Cthulhu, The Return of the Sorcerer, Ubbo-Sathla, The Black Stone, The Hounds of Tyndalos, The Space-Eaters, The Dweller in Darkness, Beyond the Threshold, The Salem Horror and The Haunter of the Graveyard.
The 1971 edition lacks an ISBN; all the other four, from 1973-1975, have them.
Volumeless editions, including the original 1969 edition, will feature more tales besides these ten. In the 1969 edition those were Robert Bloch's "The Shambler from the Stars", "The Shadow from the Steeple", "Notebook Found in a Deserted House"; HPL's "The Haunter of the Dark"; J. Ramsey Campbell's "Cold Print"; Brian Lumley's "The Sister City" and "Cement Surroundings"; James Wade's "The Deep Ones" and Colin Wilson's "The Return of the Lloigor". Several editions of Volume 2 include those and sometimes a few others.