Stoic Rebuttal’s metalcraft ability functions while Stoic Rebuttal is on the stack.

For the purpose of determining whether the cost reduction applies, the number of artifacts you control is checked as you cast Stoic Rebuttal, before your last chance to activate mana abilities to pay for it. For example, if you control three artifacts, you could determine that Stoic Rebuttal costs {U}{U} to cast, then sacrifice one of those artifacts to activate a mana ability.