The Invisibility spell (PHB, p. 254) says:
A creature you touch becomes invisible until the spell ends. Anything the target is wearing or carrying is invisible as long as it is on the target's person. The spell ends for a target that attacks or casts a spell.
And the spell Steel Wind Strike (XGE, p. 166) says:
You flourish the weapon used in the casting and then vanish to strike like the wind. Choose up to five creatures you can see within range. Make a melee spell attack against each target. On a hit, a target takes 6d10 force damage.
If a spellcaster under the effect of an Invisibility spell casts Steel Wind Strike, would they have advantage on all five melee spell attacks, or only on the first one?