"Maintaining" strictly means enchantments that require an upkeep cost of -1 energy regeneration. No other enchantments count.
Since Blessed Signet has a 2 second activation and 10 second recharge, it can be used once every 12 seconds. Over 12 seconds, one pip of energy degen will cost 4 energy. Since Blessed Signet gives 3 energy for each maintained enchantment, the caster will experience a net loss of 1 energy every 12 seconds per maintained enchantment, essentially reducing the upkeep of maintained enchantments to a quarter of a pip instead of the normal 1 pip. However, this assumes that the caster has a high enough Divine Favor to get 3 energy for each enchantment, that the signet is used every 12 seconds (with a 2 second activation, the caster would spend about 17% of the time casting), and that the signet is not interrupted.
While maintaining a large number of enchantments, this signet can be recharged faster using the Mesmer skill Mantra of Inscriptions.