Don't worry about it OP, as others have said, in dungeons you're mostly aoe-ing down packs anyway, rather than focusing one particular mob. Unless you're going absolutely ham, and purposely ignoring your aggro-reduction skills (quelling on BLM for example) it's not hard for tank to hold aggro. even if you do take aggro, just either move to a different target (if you are single targeting), or stay on that mob and kill it. tank will it either take it back or the thing will die. But try not to panic and start running about, that rarely helps. Depending what role you're playing, you'll likely have some defensive buffs you can use yourself if need be (keen flurry, manawall, etc)

no one bothers numbering targets any more, it's unnecessary micro-management