Which Mandarin learning textbooks have the largest amount of practical exercises?
You can find some bookstores near the Elementary Schools (小學).
If you see some signs like 國小參考書, go to find 參考書 (references) or 評量題 (examinations) for the first or second grade (一、二年級). 參考書 include the teachings in the textbooks and some Q&amp;A. 評量題 have only Q&amp;A.  

You should know that all of them are totally in Chinese, no English.  

(Ask your Taiwanese friends to go with you.)

You can find many Q&amp;A on 全國中小學題庫網.