The number of custom fields which can be added to Recurly records is limited by plan. For exact plan definitions, see here.
SANDBOX AND DEVELOPER MODES
In sandbox and developer modes, you can add 5 custom fields to your account. This is meant to help you test the feature and ensure that your integrations are built to use this information accurately.
Custom fields are not available in the Core plan of Recurly. If you have custom fields enabled in sandbox mode and promote to core, you will need to delete your custom fields in order to upgrade.
On the Recurly Pro plan, you can add 5 custom fields to your Recurly site.
On the Recurly Enterprise plan, you can add 10 custom fields to your Recurly site.