If I understand you correctly, a holder of several instructor ratings such as FI and TRI needs a separate refresher seminar for each rating.
No, one seminar will be all that is required for all of the Certificates held and if you hold both Aeroplane and Helicopter instructor certificates then again the same seminar may be used. In the case of adding a FI certificate that is treated seperately to the revalidation process but as long as the AoC can fill both requirements then a single test may be used. Discuss with the examiner before you do it.