I use these small, flexible cups to create highly pigmented, long-wearing cosmetics. They are heat-tolerant enough to be used in a water bath, and the small size is perfect for creating tiny batches of highly pigmented, potent things. I can “corner” whatever it is I’m making and thoroughly mix it up before scraping/pouring it out into the final container. Since the cups are a soft, flexible plastic I can gently squeeze them with my fingers to create a makeshift spout. While disposable plastic isn’t great, neither is attempting to clean a coating of waterproof eyeliner out of anything—I figure simply tossing these little cups is probably easier on the environment than using a lot of detergent & water (and ruining a sponge or two) trying to scrub out something re-useable.
How many you’ll need will depend on how often you make high pigment cosmetics. If you never make cosmetics you can likely get away with not owning any; otherwise, I’d start with 100 and see where that takes you.