If Hall of Storm Giants enters the battlefield at the same time as one or more other lands, it doesn't take those lands into consideration when determining how many other lands you control.

If an opponent casts a spell or ability that targets Hall of Storm Giants, and you activate the last ability in response, the ward ability Hall of Storm Giants gains won't trigger. (However, if it already had ward at the moment it became the target of the spell or ability, that instance of ward would trigger.)

If you activate Hall of Storm Giants's last ability multiple times, it gains multiple instances of ward {3}. Each of those instances will trigger separately and ask the opponent who controls the spell or ability to pay {3}. If they don't for any of those instances, the spell or ability will be countered.

If you turn Hall of Storm Giants into a creature but haven't controlled it continuously since your most recent turn began, you won't be able to activate its mana ability or attack with it.