Most bosses get a tank buster and a cleave. Tank busters will kill you if you arent mitigating with a cool down and nearly/fully topped off. Cleaves are frontal attacks that are aimed at you. If you stand near a party member and get cleaved, they do too and they dont have the hp or defense you do. They'll likely die.
There are other mechanics. Some bosses require tank swapping to prevent debuff stacks from getting too high on one tank.

Your best bet is to know the fight before hand. Read a guide on what moves each boss uses and remember what they do and how they need to be handled.
Because gear changes, stats changes, and party make up are never the same. We cannot tell you how many to pull. It varies. As one user mentioned, you pull packs that are tethered. You cant really pull a specific number of mobs.

I would pull small and add more until you feel that you cant safely pull any more. If the healer is sitting at full mp and not healing much, pull. If you are barely losing hp, pull. If your'e running in a pug, its best to have communication and to ask "how many?" "is this too much?" "too little?" Its not as hard as it seems. If you sneeze, you have full aggro these days. If your bar is going down faster than your healer can raise it, you pulled too much. :P