KeySeeder.cs

public class KeySeeder
{
    const int KeySize = 8;

    private Random random;
    private int currIndex = 0;
    private char[] keyBuffer = new char[KeySize];

    private char[] keyChars = "abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ-_".ToCharArray();

    public KeySeeder(int seed)
    {
       random = new Random(seed);
    }

    public string GetKey(int index)
    {
        if (index < currIndex) throw new Exception($"Index {index} is lower than current index {currIndex}");
        for(; index > currIndex; currIndex++)
        {
            for (int i = 0; i < KeySize; i++)
            {
                keyBuffer[i] = keyChars[random.Next(keyChars.Length - 1)];
            }
        }
        return new string(keyBuffer);

    }

}
    
var ks = new KeySeeder(39329);
Console.WriteLine($"Key #1: {ks.GetKey(1)}"); 
Console.WriteLine($"Key #2: {ks.GetKey(2)}"); 
Console.WriteLine($"Key #3: {ks.GetKey(3)}"); 
Console.WriteLine($"Key #4: {ks.GetKey(4)}");