From the sounds of the blog post, you can select as many of them at the same time as you want. It's just an option for people who, for example, never play PvP to not get their UI cluttered with PvP goals.

Edit: it might even go a step further and give you the same amount of goals, but spread only across the modes you selected, so everyone can complete the same amount of goals. Just speculation of course, we'll see on Tuesday.