If “Rush Recklessly” is chained to the effect of “Blast Magician”, and the ATK of the target monster is now higher than 700 x the number of Spell Counters removed, the effect of “Blast Magician” disappears. You cannot remove additional Spell Counters to increase the effect.
This is an effect that targets.
You must remove at least 1 Spell Counter. You cannot activate the effect and remove zero Spell Counters.
As with all Spell Counters, the Spell Counter is placed when the Spell Card resolves, not when it is activated, but “Blast Magician” must have been face-up on the field when the Spell Card was activated (and when it resolves) in order to receive a Spell Counter.