private void btn_cadastrar_Click(object sender, EventArgs e)
    {
        sqlcon.Open();
        string sql = "SELECT COUNT(*) FROM userDB";
        SqlCommand cmd = new SqlCommand(sql, sqlcon);
        Int32 count = Convert.ToInt32(cmd.ExecuteScalar());
        if (count == 5)
        {
            MessageBox.Show("São permitidos apenas 5 usuários", "Ficha de Anamnese - ERRO", MessageBoxButtons.OK, MessageBoxIcon.Information);
        }
        else
        {
            var newuser = new cadastrodeusuario();
            newuser.Show();
            this.Hide();
        }
        sqlcon.Close();
    }