Hi Dani,
Some manufacturers recommend the mixing in two cups because many people won’t do a thorough job mixing. They don’t scrape the sides of their cup well, but will scrape this resin out at the end to go into their mold, bezel, etc. This resin is minimally mixed, so it will cure sticky if at all. The way to avoid this is to pour the mixed resin in a second cup where it is less likely you will have unmixed resin on the side of your cup.
As for the smallest quantity of resin you can mix, that’s a great question! It actually varies from resin to resin. The specific manufacturer should be able to tell you this. For the resins we sell on Resin Obsession ( https://shop.resinobsession.com/collections/resin ), you can find it in the PDF on this page: https://resinobsession.com/resin-frequently-asked-questions/resin-casting