{counterflux}

[Rontron]

Does {counterflux} overloaded counter all spell cast the turn its cast or does it counter all spells during that step?

[Gorzo]

Counterflux counters all spells that still underneath it on the stack waiting to resolve. (That you don't control)

Nothing cast previously (before counterflux resolves) in the turn would be affected, as they are no longer spells - they are either permanents, or resolved spells that are done already and in the graveyard.

Example. You cast {Ash Zealot}. It resolves. You attack with Ash Zealot, and your opponent attempts to cast {Shock} on her. You respond with {Giant Growth}. Your opponent responds with {Lightning Bolt}. You respond with overloaded {Counterflux}. The stack now looks like this:

TOP
Counterflux (Overloaded)
Lightning Bolt (T: Ash Zealot)
Giant Growth (T: Ash Zealot)
Shock: (T: Ash Zealot)
BOTTOM

When Counterflux resolves, it counters each spell on the stack you don't control. Lightning Bolt and Shock are countered and removed from the stack.

Giant growth resolves still, because you controlled it! Your now 5/5 ash zealot is attacking.

RESOLVED