How to get value from wp_usermeta table in database?

You should be using add_user_meta instead of update_user_meta:

if (isset($_POST['billing_cpf'])) {
    add_user_meta($customer_id, 'billing_cpf', $POST['billing_cpf'], true);
}

As this will only add the meta if it does not already exist – no update.

(This will prevent this field from being updatable – unless by you another way.)

Note: normally you could use something like this to add and update in combo so that the field is updatable:

if (isset($_POST['billing_cpf'])) {
    if (!add_user_meta($customer_id, 'billing_cpf', $POST['billing_cpf'], true)) {
        update_user_meta($customer_id, 'billing_cpf', $POST['billing_cpf']);
    }
}