Blizzard’s profession overview allows two primary professions and all three secondary professions. Each primary profession takes one slot, whether gathering or crafting.
Which professions count towards the limit?
The gathering professions Skinning, Mining and Herbalism are primary professions. So are the crafting professions Alchemy, Blacksmithing, Enchanting, Engineering, Leatherworking and Tailoring.
Cooking, Fishing and First Aid are secondary professions. You can learn all three alongside your two primary choices.
Three combinations that fit
Herbalism + Alchemy fills both primary slots. Mining + Skinning also fills both. Tailoring + Enchanting does too. Any of these combinations can add Cooking, Fishing and First Aid.
These are examples of how the slots work. They are not rankings or claims about income, levelling speed or the best choice for your class.
Where do I start?
Learn a profession from a trainer in the world or a major city. Open the Professions tab (K) to see recipes, required materials and whether crafting an item can increase your skill. Ranks progress through Apprentice, Journeyman, Expert and Artisan.
Make a small plan before spending
Choose one thing you would enjoy doing: gathering while exploring, making an item you want, or contributing something your friends have asked for. Write that goal down before comparing combinations.
Look at the materials for your first crafting goal. Decide what you are comfortable spending and where you want to stop for the evening. If the goal looks too large, choose a smaller one rather than assuming you must finish a whole levelling plan.
Planning to gather first and craft later? Count the primary slots at both stages. If both are occupied, adding a different primary profession means replacing a choice. Check what happens to your skill and recipes before dropping a profession.
Once you understand the slots, choose what the professions will add to your evenings.