int[] maxNumbers = new int[10];
for (int i = 0; i < mas.GetLength(0); i++)
{
    for (int j = 0; j < mas.GetLength(1); j++)
    {
        if (maxNumbers[i] < mas[i, j])
        {
            maxNumbers[i] = mas[i, j];
        }
    }
}
foreach (var maxNumber in maxNumbers)
{
    Console.WriteLine(maxNumber);
}