is the sentence 她做练习做的很多 correct?
她做练习做得很多 is grammatical. 得 (degree particle) should be followed by a degree and 很多 is indeed a degree.
Strictly speaking, 得， 的 and 地 have different functions, and should not be confused,  however, it has become a common practice for many Mandarin speakers to use them all interchangeably, so 她做练习做的很多 or 她练习做的很多 is also acceptable because 的 here is not a noun modifier but a replacement of 得 (verb modifier)