The relevant part of Darksteel Juggernaut's rules text states:
Darksteel Juggernaut's power and toughness are each equal to the number of artifacts you control.
When is Darksteel Juggernaut's power and toughness determined? Can they change?
I am not a judge, but I believe the following sections from the comprehensive rules cover everything:
And if you check the card rulings for Serra Avatar (with the same ability), it has the following ruling:
So, its power and toughness are always defined, in all zones, and always updated. If anything needs to reference its power and toughness, determine what that is. If a new artifact comes into play, it automatically goes up in power and toughness.
I haven't played MtG in quite a while, but I would assume the power and toughness change as the number of artifacts you control changes. For example, if you had no other artifacts in play when you played Darksteel Juggernaut, it would come into play as a 1/1 (I'm assuming an Artifact Creature counts as an artifact). If you later played two more artifacts, it would become a 3/3. So basically, whenever you need to know what the power or toughness is, you count the number of artifacts at that time.