Six cards in total! https://www.hearthpwn.com/news/5553-hearthstones-next-balance-update-goes-live-on-may
For those out of the loop, here's the nerfs that are coming in the update.
Naga Sea Witch - Cost is now 8, up from 5.
Spiteful Summoner - Cost is now 7, up from 6.
Dark Pact - Now restores 4 Health, down from 8.
Possessed Lackey - Cost is now 6, up from 5.
Call to Arms - Cost is now 5, up from 4.
Crystal Core - Effect now makes all minions in your deck 4/4, down from 5/5.
RE: High Legend Hearthstone: Nerfs Are Live, Let's See What Decks Are Still Good