I was wondering if I can use the Darkvision spell on more then one person? Or if I cast it on someone else does it cancel the other one out. I have people in my group that(you guessed it) can't see in the dark, and if I can cast it on both of them that would be very helpful. It doesn't say in the rules that I can't cast it on one person then another. 

Darkvision does not have a notation that it is a Concentration spell so, by my understanding, you can cast it on multiple people so long as you have the resources.

That means, in your case, the Monk must spend 2 Ki points per "casting" of Darkvision.

Ok that works, because I am a dark elf monk so I get the spell for being a dark elf as well and can use that for Both people using the materials so I can save my ki points for something else. Just a pinch of dried 🥕.

Darkness does have a duration of 10 minutes with Concentration, so you can only have 1 up at a time. https://www.dndbeyond.com/spells/darkness

It's main limiter, it keeps you from using other great concentration spells like Hex or whatnot at the same time.

As a Drow you know Darkness at 5th level and can only cast it once per day. It is also a Concentration spell which means you are limited to one Darkness, cast by you, at any given time.

As a Way of the Shadow Monk you get Darkness, Darkvision, and a couple others which can be cast as many times as you have enough Ki points.

With the Drow spells you would need an Arcane Focus or Component Pouch. If you have an Arcane Focus then you wouldn't need the spell components.

With the Way of the Shadow Monk you would use the Ki points in lieu of any spell components.