A fighter/paladin/swordmage can only mark one target at a time (not counting a special specific power/ability). A warden can mark multiple targets at a time.
Incorrect.
1) Marked is a condition (see p277) that has a special rule that only one creature can mark you at a time. The condition applies: 'You get a -2 penalty with any attack that does not include the creature that marked you.'
That is it. Nothing else. Nothing restricts -any- character from marking multiple creatures.
2) The fighter and the warden have ways to apply the marked condition that are outside of the use of powers. The fighter may apply it to -any- creature he's attacked, and the warden can apply it, once per round, to all adjacent creatures. This ability allows the fighter to mark multiple targets, as well as the warden. (Example: A Dragonborn uses Dragon Breath to mark multiple targets in the blast. A tempest fighter uses Dual Strike to mark two opponents.)
3) The fighter and the warden also have class features that allow them to react to creatures with the marked condition. How that creature got marked to them is irrelevant (Misdirected Marks from a Bard count, as do marks from other class features/powers, say from multiclassing).
4) The paladin and swordmage have class feature powers that assign a single target a mark, and a triggered condition for a punishment. That's -two- effects, not one. It's mark+something else. Now these powers -explicitly state- that applying -that power- to another target ends the effects on the first target. This has nothing to do with marks tho, this is based on using the power. Divine Challenge doesn't care that you've -marked- someone else, it cares if you've Divine Challenged someone else.
5) The paladin and swordmage have attack powers that explicitly mark multiple creatures as part of their attacks. (Q.V. Arcing Smite, PHB1, p 93) These marks have nothing to do with Divine Challenge OR Swordmage Aegis. If the marked creature doesn't attack the Paladin or Swordmage, then the defender doesn't get to apply his Divine Challenge/Aegis, because those are the additional effects of powers, and are not the result of marking an opponent.
So to summarize:
Fighters and Wardens apply the marked condition, and have abilities that trigger off the condition itself.
Paladins and Swordmages have powers that apply the marked condition, and some of those powers -may- include a punishment for the target, but apply no punishments regarding the marked condition itself.
Regardless, this means one thing. -Any- defender can mark multiple opponents, if they have the means to do so, and with impunity. A Paladin or Swordmage cannot Divine Challenge/Swordmage Aegis multiple opponents with their basic powers.
The part about a creature only having one mark at a time on it tho? That is correct.