---

int[] arr; // поле класса — вне методов!

private void button5_Click(object sender, EventArgs e)
{
    arr = new int[10];
    Random r = new Random();
    textBox7.Clear();
    
    for (int i = 0; i < arr.Length; i++)
    {
        arr[i] = r.Next(10, 100);
        textBox7.Text += arr[i].ToString() + " ";
    }
}

private void button4_Click(object sender, EventArgs e)
{
    int x = Convert.ToInt32(textBox8.Text); // textBox8 — для ввода числа x
    int count = 0;
    
    foreach (int num in arr)
    {
        int sum = num / 10 + num % 10;
        
        if (sum == x)
            count++;
    }
    
    textBox9.Text = "Количество = " + count;
}