In the WOW GOLD, there are two types of professions, primary and secondary. Every player can only choose two Primary Professions but may have as many Secondary Professions as he or she desires. This passage we only discuss the ways to choose primary professions.
You can only have two primary professions at a time. But they are not fixed, you can give up a primary profession and replace it, and they are especially cheap. When you just begin to play, you might benefit by switching professions to meet a goal. You can use this to get a fair upgrade to your starting gear cheaply, and stock up on some low level consumables. While the disadvantage is that every time you switch professions, you will lose all of your built up skill and recipe knowledge in the one you drop. Your professions provide more capability and benefit when you keep them leveled up as you progress through the game. So in order to quick level our characters, it's better for us to cherry pick the professions all at once.