Luminate Email will not send a message with more than 255 merge fields (S-tags, URLs, conditionals, in both message body and stationery). There is no in-product dialog that notifies the user when they have exceeded 255 merge fields, though. 15.9.7 adds the words that there is a content error when the user tries to send a Test Message, and the new documentation notes this limit, but it would save time for the admin and Support tracking down the content error if Luminate would notify the admin specifically that they have exceeded the number of merge fields when trying to save their email draft.