[Python 2], 104 94 89 81 bytes

s=n=input()
while s:print' '*s,'/ '*(n-s)+'_'*(2*s+len(`n`));s-=1
print'\/'*n,`n`

[Try it online!][TIO-ka21f5ec]

Edit 1: Forgot I switched to python 2 lol

Edit 2: Thanks @ElPedro for the idea of switching to while loop!

Edit 3: Thanks @SurculoseSputum for saving 8 bytes! [Python 2]: https://docs.python.org/2/ [TIO-ka21f5ec]: https://tio.run/##JY3BCsIwEETv@xW5bTZtqYn0UsmfCBbqQgNlG9yI@vUx4mmGx/Amf8p2SKjrcWcTDSJWjRKT5GexBK8t7Wx0zo8kBQ067XFsYWVQ6vDWWnDa7Sx2kYXookP08F9fR3TSN1ybFfjNq/29uImqhwBnmMCfvg "Python 2 – Try It Online"