As many as are necessary. The main difference is that even with sharding, some people are going to fly through it, some people are going to corpse run to another starting zone because they want to, and some will take their time.
Some people won’t even bother getting out of the starting zone because they don’t like the feel of Classic.
The zones after the starting zones are larger, there is a larger pool of quests available, and there won’t be as many people in the same tiny spot at the same time. Sharding launch areas is a necessary evil. Beyond that, not so much.
YES SHARDING.
Is terrible.
Good.
More than you might think.