According to a paper titled Nietzsche's Reception of Buddhist Psychology With Constant Reference to Christianity by McDonald (2012), given at a conference in Copenhagen, Nietzsche's work contains 158 references to Buddhismus and its cognates within 137 textual units in the collected Digitale Kritische Gesamtausgabe of Nietzsche’s Werke und Brief.
Nietzsche's engagement with the thought of Schopenhauer in 1865 seems to have been the main stimulus for his interest in Buddhist and Brahman texts, interestingly the article cites that Nietzsche's library and reading included:
Otto Böhtlingk’s Indische Sprüche, 2nd ed. 3 volumes. Paul Deussen’s Die Elemente der Metaphysik, (Aachen, 1877); Das System des Vedanta (Leipzig, 1883) and Die Sutras des Vedanta aus dem Sanskrit übersetzt (Leipzig, 1887); Max Müller’s Essays, II. Beiträge zur vergleichenden Mythologie und Ethnologie (Leipzig, 1869); Jakob Wackernagel’s Über den Ursprung des Brahmanismus (Basel, 1877); Hermann Oldenberg’s Buddha. Sein Leben, seine Lehre, seine Gemeinde (Berlin, 1881); and Louis Jacolliot’s Les legislateurs religieux. Manour-Moiser-Mahomet (Paris,1876). He also borrowed, from the university library in Basel, Martin Haug’s Brahma und die Brahmanen twice (in 1873 & 1879), and the two volumes of Carl F. Koeppen’s Die Religion des Buddha in October 1871.
The foundation of his critique is his characterisation of Nirvana (as a nothingness) as a form of nihilism. According to McDonald this is the result of having been introduced to Eastern thought through the works of Schopenhauer, who in his conflation of Buddhist and Brahman notions incorporates the tenets of both into his conception of the will as a chaotic, aimlessly striving unity.
Both Nietzsche's and Buddhist writings share the fact that they are a direct response to nihilism, however was he right in characterising Buddhism as advocating a negation of the will, as a will to nothingness, or was this a misunderstanding stemming from his reading of Buddhist texts through the works of Schopenhauer?