Just for a heads up, I found
this site saying that a drop of water is approximately 0.05ml per drop. This would mean that to fill a volume of a coffee mug (300ml), you would need at least 6000 of these just to fill a cup.
@LordXCano: You would be very surprised. Considering the size of those raindrops and the spacing between them, I am leaning towards way more than 20800, or even 108000 raindrops for that matter.
The volume of a hemisphere that was a 12 foot radius. The volume of a hemisphere that has a 12 foot radius hemisphere has a volume of about 97.7m^3, or 9.77e10 mm^3. Considering each and every drop only takes about 0.05ml, it would take approximately 1.954e12 raidrops to fill that area.
Also keep in mind that for beings like Dante/Vergil, they are considered to be far and beyond average humans; this means their sense of perception is likely insanely higher compared to ours, meaning any limitations that an average human might face flies out the windows for something like this. Even the raindrops that would be completely invisible to humans would be completely visible to a person like Dante/Vergil.
Redgrave is correct on this one.
Lastly, Alakabamm has made a commenting regarding your suggestion in regards to Dante cutting more than one raindrop in the video, as the video never explicitly stated that only one raindrops were cut. I will take your/Cross' suggestion in regards to the sword cutting more than one raindrop however.