Button incrementing from 0 to 9 and repeating
Your code is ok. But there are always other ways to get the same result which may be more elegant.

private void counterButton_Click(object sender, EventArgs e)
{
    szám=(szám +1) % 10;
    this.Text = szám.ToString();
}


or you can do:

private void counterButton_Click(object sender, EventArgs e)
{
    szám=(9==szám)?0:szám+1;
    this.Text = szám.ToString();
}