[2022] Can we get information on Vaal flicker strike, if used on a single monster, does it count as 25 hits or as 1 big hit that would be the result of 25 hits added together ?

Calculating the damage of a hit inherently also calculates the damage of the ailments it will inflict. This combines damage from multiple calculated hits in the exact same way as Dual Strike, Cleave, and (most similarly) Explosive Arrow.

You hit each enemy once each time you wound them, calculating the damage of the hit (including damaging ailments) but not applying that damage. You are still hitting, so do get all on-hit effects that don't require damage.

At the end, for each enemy, you hit them an additional time, causing on-hit effects again, and dealing the combined damage of all the wounds applied to that enemy as a single hit. If those hits calculated damaging ailments, the damage of those ailments is merged the same way. If some of those hits were crits, then the critical multiplier will have applied to those specific hits when they calcualted, and will not apply to others, but if any of the combined hits was a crit, the combined hit counts as dealing a critical strike for things that care about that.

Just like the hit damage is combined into one hit, the associated damaging ailments from those hits are combined into one ailment of each damaging ailment type - if multiple of the wounding hits calculate an ignite, the total ignite fire damage per second from all such hits will be combined into a single ignite applied by the final damaging hit. The same is true of other damaging ailments.

Just to confirm, does this mean that if I link Vaal Flicker Strike to Curse on Hit Hextouch, then the initial hits causing wounds will apply the curse to all enemies hit

Yes

and the final hit that deals damage can benefit from the effects of this curse?

That depends what you mean by "benefit from". The damage was already calculated, so anything the curse does to affect damage calculation, such as resistance penalties, can't apply unless the enemy already has the curse when wounding hit occurs. As noted above, this includes damaging ailments, which are part of damage calculation, but also notably includes stun.

But curses adding on-hit effects, including non-damaging ailments, such as Frostbite's chance to freeze, will affect what happens when actually dealing the combined damage.

No. Impale is an on-hit effect, not part of damage calculation.

Damage calculation determines:

• The amount of hit damage of each type for the hit.
• The damage per second of each damaging ailment applied by the hit.
• Any stun caused by the hit.

Anything else that happens as a result of being hit, including non-damaging ailments, is part of applying damage, not calculating it, so only happens when the damage is dealt.

Any use of a skill is a single action, no matter how long that use takes or how many times it repeats.

No, only the final hit actually inflicts bleeding on the target, so only the final hit inflicts Rupture.