When you pull one enemy, all mobs near it will get aggro as well. Most of the time you won't be able to pull precise numbers of mobs. Rather, you'll be pulling precise packs of them.

My general rule has been if you're new to a dungeon just say so. You may even have someone with plenty of tank experience leveling another class. They will be invaluable. Aside from that, if you're unsure about a dungeon, try pulling only the first pack of mobs and gauge it from there. If the damage is negligible then try pulling two packs. It's when you start going wall to wall that things can get sketchy.

Normal enemies tend to not have any tank busters. You'll just need to learn the ones that do. Practically all bosses have one or more tank busters. You'll just need to recognize the skill name when the boss begins to cast it. For casual content it's not so bad so long as you keep a defensive skill up and not pop them all at once.

Good luck and happy tanking.