List<T>.Insert() / Count

using System.Collections.Generic;

// Inserts an element at the specified index.
list.Insert(int index, T item)

// Inserts all elements of a collection at the specified index.
list.InsertRange(int index, IEnumerable<T> collection)

// Gets the current number of elements in the list.
list.Count

using System;
using System.Collections.Generic;

// Create a List<string> with initial values.
List<string> ranking = new List<string> { "Tanaka", "Sato", "Suzuki" };
Console.WriteLine($"Count: {ranking.Count}"); // Count: 3

// Use Insert() to insert at index 1.
ranking.Insert(1, "Yamada");
Console.WriteLine(string.Join(", ", ranking)); // Tanaka, Yamada, Sato, Suzuki
Console.WriteLine($"Count: {ranking.Count}"); // Count: 4

// Insert at the beginning (index 0).
ranking.Insert(0, "Ito");
Console.WriteLine(string.Join(", ", ranking)); // Ito, Tanaka, Yamada, Sato, Suzuki

// Use InsertRange() to insert multiple elements at once.
List<int> numbers = new List<int> { 1, 5 };
numbers.InsertRange(1, new int[] { 2, 3, 4 }); // Insert at index 1.
Console.WriteLine(string.Join(", ", numbers)); // 1, 2, 3, 4, 5

// Example of using Count in a loop.
List<int> evens = new List<int>();
for (int i = 2; i <= 10; i += 2)
{
    evens.Add(i);
}
Console.WriteLine($"Even count: {evens.Count}"); // Even count: 5

Count is the list equivalent of the array Length property. Because List<T> does not have a Length property, calling Length by mistake — confusing it with an array — will cause a compile error. Always use Count to get the number of elements in a list.

Insert() must shift all elements after the insertion point, so the larger the list, the higher the processing cost. If you frequently need to insert at the beginning or in the middle of a collection, consider using LinkedList<T> instead.

For searching and checking elements, see 'List<T>.Contains() / IndexOf()'.