You may take as many Merwyrms as you have Storm Weavers and Mist Mages in the army subject to the restrictions on rare units. Each Merwyrm is 'bound' to a specific mage before deployment (note this on your army list).

In any turn in which a Merwyrm fails a roll to Regenerate, it must make a Leadership test, using the Leadership of the mage that has bound it. If the test is failed, roll on the Monster Reaction table (regard result 5-6 as the creature acting really dumb rather than guarding a fallen rider!).

Should the mage be slain, the Merwyrm must test on its own Leadership each turn, and if it fails, the effect of the roll on the Monster Reaction table will last until the end of the game.