SQL command not executing as expected

cmd.CommandText = "select max(remain) from item_new_customer where cust='" + textBox2.Text + "'";

You are open for SQL-Injection, use parameters to avoid it.

To answer your actual question, i assume that you want this column: remain. But you want the value of the last inserted record. Since you haven't mentioned the column to detect the order of insertion, i use the primary key column (not recommended):

string sql = "SELECT TOP 1 remain FROM dbo.tablename WHERE cust=@cust ORDER BY id DESC";
using(var con = new SqlConnection(connectionString))
using(var cmd = new SqlCommand(sql, con))
{
    cmd.Parameters.AddWithValue("@cust", textBox2.Text);
    con.Open();
    double h = (double)cmd.ExecuteScalar();
    textBox20.Text = h.ToString();
}

You're missing a single quote after the textBox2.Text:

private void textBox2_Leave(object sender, EventArgs e)
{
    cmd.Connection = cn;
    cmd.CommandType = CommandType.Text;


    cmd.CommandText = 
   "select max(remain) from item_new_customer where cust=" + textBox2.Text + "'";
                                                          //Missing tick here ^ 
    float h = (float)cmd.ExecuteScalar();
    textBox20.Text = h.ToString();
}

In addition, your code is an open invitation for SQL injection.

Thanks very much for all My final right code is:

cmd.Connection = cn;
        cmd.CommandType = CommandType.Text;
        cmd.CommandText = "select top 1(remain) from item_new_customer where cust='"+textBox2.Text+"' order by id desc";
        int h = (int)cmd.ExecuteScalar();
        textBox20.Text = h.ToString(); 

What I would just do is put all of the SQL row values into a listbox and then take the text of the last item in the textbox and put that into a textbox. Keep the listbox hidden

    private System.Windows.Forms.ListBox listBox1;
    static SqlConnection connection = new SqlConnection(@"Data Source=hostname;Initial Catalog=database_name;Integrated Security=False;User ID=user;Password=123456;");
    SqlDataAdapter adapter = new SqlDataAdapter("select * from table_name", connection);
    DataTable table = new DataTable();
    adapter.Fill(table);
    foreach (DataRow row in table.Rows)
    {
    listBox1.Items.Add(row["row_name"].ToString());
    }
    textBox20.Text = listBox1.Items[listBox1.Items.Count - 1].ToString();