Source: Player's Guide to Faerûn
You can counterspell any number of spells each round.
Prerequisite: Spellcraft 30 ranks, Combat Reflexes, Improved Counterspell, Improved Initiative, Quicken Spell, Reactive Counterspell.
Benefit: Any number of times per round, you can counterspell an opponent's spell even if you have not readied an action to do so. Such a counterspell doesn't count against your later actions for the round. You can even use this feat when flat-footed.
Special: This feat should be considered part of the epic bonus feat list for any class that grants access to spells of at least 6th level as part of normal (non-epic) class progression (such as bards, clerics, druids, sorcerers, and wizards), as well as for any prestige class that grants a "+1 level of existing class" spell progression at all levels.
Normal: Without this feat, a character must ready an action in each round that she wishes to use a counterspell. A character with the Reactive Counterspell feat can counterspell an opponent's spell if she hasn't readied an action, but only once per turn and not when flat-footed.