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dic.yahoo.co.jp、weblio.jp、and other dictionaries, state that the parts of speech for「たくさん」are noun and 形容動詞。imho, usage cases where たくさん is used as 形容詞 is uncommon. I have seen たくさん take the な particle in sentences such as "写真{しゃしん}がたくさんな記事{きじ}" . But, I have never seen たくさん take the に particle. ...