Merit once to unlock merit two more times to cap it out for a total of +12% Max HP Bonus to Party(Not Alliance) Members in Range of Effect for a Duration of 3*mins. Nice prep JA for an HNM fight but still very limited otherwise. So its really not worth the merits considering you could Merit "Formless Strikes" and "Penance" which are the best merits Monk has now. Even "Invigorate" is better than this merit. Invigorate can help you solo a ton easier as it ADDS 240hp "fully merited" to chakra. Formless Strikes bypasses Invincible as well as increases your dmg against mobs weak to magic and then Penance which allows a Monk to prevent the mob from gaining TP for a certain amount of time, With enough Monks this lil Merit can keep Mobs from building TP indefinatly.
Above poster is right. You can only add three merits total into one new merit category. Additionally, the maximum number of cumulative merits is six. Therefore you can only fully merit two of these new categories.
And Mantra is HP boost of 4% per merit. It can potentially add much more to your Hit Points than a mere 71 if you activated it while wearing AF or AF2 gear. The same goes for party members.