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\usepackage{enumitem}
\setenumerate{label=\arabic*}
\setenumerate[2]{label=\alph*)}
I imagine that after 26 letters of the alphabet are used, obtions aa), ab), ac).. az), ba), bb), bc) etc. etc. follow. Then if a very long list (more than 26^2 elements), aaa), aab), aac) etc. should follow.