Is this correct usage of the word 天资?
For talent in a specific area, the word 天份 is better choice than 天资, which can also means 'talent' but in a more general sense. When you say someone has 天份(talent), you need to specify in what subject the talent applies; When you say someone has 天资, it means that person has a nature ability to learn, doesn't matter what is the subject.

We don't measure 天份' with '多 and 少',  We measure '天资 or 天份' with '有 and 沒有' ; '高 and 低'


有天份; 沒有天份 (have talent/ talented; have no talent)
天份高: 天份低 (highly talented; low in talent/ not very talented)



  弹钢琴他有很多天资 should be written as "弹钢琴他很有天份" which is a [topic]+[comment] sentence structure with "弹钢琴" as the topic and " 他很有天份" as the comment.


For a more common [S+V+O] structure, the sentence should be written as " 他很有弹钢琴的天份"


  Is this correct usage of the word 天资?


Example of correct usage of the word 天资:

"他天资过人，學一年就能弹钢琴" - His talent is extraordinary, can play piano after only a year of learning.

"凭他的天资，考上多伦多大学不成问题" - With his talent, be admitted to the University of Toronto is not a problem.