One article I read said 300 times, but I have read the book several times and I don't think the word appears that many times -- I believe that was an exaggeration for shock effect.
Even if I'm wrong and the word does appear that many times, it's important to keep the context in mind; the book was set in the deep south during the 1930s when Jim Crow (anti-black) laws and traditions were commonplace and the casual use of the word was (unfortunately) typical. The book's dialogue is historically accurate.
More importantly, though, the book was a strong stand against racism: the themes of the book are all about racial and human equality. Written in the early 60s as the Civil Rights Movement in America was beginning to stir, To Kill a Mockingbird exposed the hatred of racism as being cowardly and low-class. The N-word has become a despised and forbidden word over the decades since the 1960s, because our sensitivities have been awakened. This is a positive (though small) step in the right direction; however, when the word appears in literature reflecting earlier times in history, we shouldn't 't blot it out, censor it, and forbid the literature, in my opinion. We should certainly cringe at the ugliness of the word, and discuss the inappropriateness of the word with each other, but we shouldn't pretend it never existed.
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There's an online version linked to in the related links section with a search function. Using it found 380 occurrences of (the word referred to that WikiAnswers will not allow me to use within an answer) within the book. Some of these may have been in illustration captions or chapter titles, as, er, "N-gg-r Jim" is a major character in the novel.
The word appears in "To Kill a Mockingbird" 48 times. Its use in the book reflects the racial prejudices and attitudes of the time in which the story is set.