root of root of root of root
Python 2, 104 94 89 81 79 bytes

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

Try it online!
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!
Edit 4: Thanks @xnor for saving 2 bytes!