Majestic Myriarch's power and toughness are each equal to twice the number of creatures you control.
At the beginning of each combat, Majestic Myriarch gains flying until end of turn if you control a creature with flying. The same is true for first strike, double strike, deathtouch, haste, hexproof, indestructible, lifelink, menace, reach, trample, and vigilance.
7/14/2017 Majestic Myriarch’s first ability applies in all zones. While it’s on the battlefield, it counts itself, so it’ll be at least 2/2.
7/14/2017 Majestic Myriarch’s second ability triggers at the beginning of each combat, not just combat on your turn, whether or not any creatures you control have any of the listed abilities.
7/14/2017 Which abilities Majestic Myriarch gains is determined as the ability resolves. They won’t change even if every other creature that has the abilities leaves the battlefield or if creatures enter the battlefield or gain abilities.
7/14/2017 If a creature gains one of the listed abilities before Majestic Mryiarch’s triggered ability resolves, perhaps due to another ability that triggered at the beginning of combat, Majestic Myriarch will gain that ability.
7/14/2017 Multiple instances of any of the abilities Majestic Myriarch can gain are redundant.