If you want to initialize a list of fixed size n, how should we do this in C#?

First, you should consider just use array. After all, why bother use List if you already know how much elements are there? I mean, you should use List when you don't yet know how many elements are to be...

int[] arr = new int[n];

However, if you insist, here is how:

// most straightforward way 
List<int> L = new List<int> (new int[n]); 

Side remark

Be cautious, new List<int> (n) will give a list of capacity n, instead of a list of length n!

using System;
using System.Collections.Generic; 

public class Program
{
    public static void Main()
    {
        int n = 10;
        List<int> L = new List<int> (n);
        Console.WriteLine("Count = " + L.Count); 
        Console.WriteLine("Capacity = " + L.Capacity); 
    }
}

This gives:

Count = 0
Capacity = 10

Reference

https://stackoverflow.com/questions/466946/how-to-initialize-a-listt-to-a-given-size-as-opposed-to-capacity