In 虽然练习做 ( ) 很多，但是我还是不懂, should we use 的 or 得？
In this context 得 should be used. It works as a degree complement, and takes as a form of "verb + 得 + adjective". The phrase after 得 expresses the degree of the adjective.

So in this case,


  虽然练习做得很多，但是我还是不懂。


, where 得 expresses the degree of "做" is "很多".

You shall also refer to this question, which explain the difference betwen 的, 得, and 地.