C# (Visual C# Interactive Compiler), 217217 154 bytes
Func<int,string>f=n=>String.Joinn=>Join("",Enumerable.Range(0,n).Select(i=>new StringS(' ',n-i+1)+new StringS('/',(n+i-n)*2i*2)+new StringS('_',(n-i)*2+1+*2-~(n/10n+"").Length)+'\n').Concat(Enumerable.Repeat("\\/",n))).Replace("//", "/ ")+" "+n;"+n
Edit: Removed 19 bytes thanks to @KevinCruijssen, and used the header better(?)