Basically you can have 2 primary crafts and all the secondary crafts that are available. As far as I know Cooking, Fishing and First Aid are the only secondary crafts there are, all other crafts fall into the 'Primary type' catagory. The skills cap out depending on the particular trades skill level and what training you have recieved in that trade.
Aprentice training caps at 75.
Journeyman training caps at 150.
Expet training caps at 255.
Artisan training caps at 300.
You can 'unlearn' your primary crafts, however, you loose all skill in that craft when you do. It is not possible to max a skill, switch it out to learn another then go back to it later and still be maxed in it. If you unlearn your skill it resets it back to 1 if you relearn it again later.