Well, you can ask LINQ to do the looping for you:
List<int> x = Enumerable.Repeat(value, count).ToList();
It’s unclear whether by “default value” you mean 0 or a custom default value.
You can make this slightly more efficient (in execution time; it’s worse in memory) by creating an array:
List<int> x = new List<int>(new int[count]);
That will do a block copy from the array into the list, which will probably be more efficient than the looping required by ToList.