28 you choose what your minion attacks, there are abilities that change what you can attack though like taunt or stealth.
you first need to get rid of all the minions with taunt before you can attack minions that don't have taunt or the opponent and minions with stealth can't be attacked but they loose it as soon as they do damage to something.
Then there is also a card that gives every minion a 50% chance to attack the wrong target and this doesn't care about taunt rules (most likely also not stealth rules) and some minions have this build in
I also have the feeling that hearthstone decks have a lot more aoe then in mtg especially since they only have 30 cards (then again they don't have to care about lands)
another thing that is different in hearthstone and mtg is the amount of RNG, in heartsthone you even have a card that costs 2 mana and puts a random card that is in the game in your hand and makes it cost 3 less. You will never see cards like that in mtg for obvious reasons