Tishana, Voice of Thunder's power and toughness are each equal to the number of cards in your hand.
You have no maximum hand size.
When Tishana enters the battlefield, draw a card for each creature you control.
9/29/2017 The number of creatures you control is counted only as Tishana’s last ability resolves. If Tishana is still on the battlefield, it’ll count itself.
9/29/2017 If Tishana enters the battlefield while you have no cards in hand, it will be put into your graveyard for having 0 toughness before its triggered ability resolves.
9/29/2017 Because damage remains marked on a creature until it’s removed as the turn ends, the damage Tishana takes during combat may become lethal if cards leave your hand later in the turn, such as by casting them in your postcombat main phase.