Darksteel Juggernaut's power and toughness are each equal to the number of artifacts you control.
Darksteel Juggernaut attacks each combat if able.
Since Darksteel Juggernaut is itself an artifact, its power and toughness will always be at least 1 while it’s on the battlefield (unless a spell or ability somehow changes its card type).
Lethal damage and effects that say “destroy” won’t cause Darksteel Juggernaut to be put into the graveyard. However, it can be put into the graveyard for a number of other reasons. The most likely reasons are if its toughness is 0 or less or it’s sacrificed.
If, during your declare attackers step, Darksteel Juggernaut is tapped, is affected by a spell or ability that says it can’t attack, or is affected by “summoning sickness,” then it doesn’t attack. If there’s a cost associated with having Darksteel Juggernaut attack, you aren’t forced to pay that cost, so it doesn’t have to attack in that case either.
The first ability works in all zones.