static int CountMinesLeft(string[,] board, string[,] visibilityArray, int count)
{
    if (visibilityArray.Length == 81)
    {
        count = 10;
    }
    else if (visibilityArray.Length == 256)
    {
        count = 40;
    }
    else if (visibilityArray.Length == 480)
    {
        count = 99;
    }

    for (int i = 0; i < board.GetLength(0); i++)
    {
        for (int p = 0; p < board.GetLength(1); p++)
        {

            if (visibilityArray[i, p] == "2")
            {
               count--;
            }

        }
    }
    return count;
}

static int CountMinesLeft(string[,] board, string[,] visibilityArray, ref int anotherCount)
{
    if (visibilityArray.Length == 81)
    {
        anotherCount = 10;
    }
    else if (visibilityArray.Length == 256)
    {
        anotherCount = 40;
    }
    else if (visibilityArray.Length == 480)
    {
        anotherCount = 99;
    }

    int count = anotherCount;
    for (int i = 0; i < board.GetLength(0); i++)
    {
        for (int p = 0; p < board.GetLength(1); p++)
        {

            if (visibilityArray[i, p] == "2")
            {
               count--;
            }
        }
    }
    return count;

int anotherCount = 0;
int count = CountMinesLeft(board, visibilityArray, ref anotherCount);