I was wondering, does passives skills take a slot in your 5 abilities bar ? or the bar is just for 5 active skills and you can stack passives as much as you can ?

The action bar is only for actives. Passives generally only apply under certain conditions, like when you have a certain weapen equipped, or do a certain type of damage.

Can see here: http://tamrielfoundry.com/dragonknight/ in the DK skills, how the passives only apply when certain conditions are met.

Same goes for the weapon lines as well. http://tamrielfoundry.com/weapons/ The passives only work when you have that weapon equipped.

you can stack as many passive as you can, provided you meet the passive requirements.

You dont need to slot them at all