re.sub and re.subn

Question

I want to use re.subn in my program to get no of times changes are made but I can't figure it out how to write the code. I have written the program with re.sub and it works fine but If I use (re.sub(r'\bi\b','I',contents)) in this program it shows this error.

TypeError: write() argument must be str, not int

---

import re

with open ('Sample.txt','r') as rf:
    contents=rf.read()
    change=(re.sub(r'\bi\b','I',contents))
    with open ('Sample2.txt','w') as wf:
        wf.writelines(change)

Can someone please guide how can I use re.subn to get the count. Thank you.