The amount of ink used in a tattoo can vary drastically depending on the size of the tattoo, its coverage, and what colours are used.
You want to avoid unnecessary waste by not pouring too much into the ink caps, as you’ll need to throw away any ink that’s been poured and not used. However, this needs to be balanced against the hassle of adding more ink to your caps, which requires removing your gloves, a thorough handwash, and another pair of gloves to keep things sterile.
Tattoo ink will go a lot further than you first expect, as only a small amount can be drawn into the needle or cartridge anyway, but with time you’ll learn to estimate how much ink you’ll need in your caps. Also keep in mind that with every dip of the needles into the cap, you’ll be diluting the ink slightly.