The above cutaway diagram of a punched cartomizer shows both the airflow and juice flow of a cartomizer in a tank. The red arrows show the juice flow. The suction from a draw from the drip tip pulls juice down to the heating coil in the cartomizer, and also from the tank. This is why even if completely saturated when you start out with a new carto, after a few vapes the top of the filler will appear dry (which is normal; now you know why). As long as the filler around the heating coil stays moist, it will not burn. The filler material does not absorb e-liquid like a sponge absorbs water, so once you begin vaping the level of the juice in the carto will fall to a certain point and remain at that level. Don't expect the tank to keep the cartomizer as full as when you first start out with a fresh tank/carto.
There are minor differences in the different brands of cartomizers. One of them is the density of the filler material. Boge's typically is more dense, slower to fill and wick, but less likely to flood. Smoktech's typically is looser fill, easier to fill and and wick but possibly easier to flood. Choose your cartomizer according to your vaping style. Number of holes required will depend on juice viscosity (thickness), clear or dark flavor, and draw technique.
Heavy vapors or with a hard draw technique may find that they have issues with either dry hits or flooding. Cartotanks work best with a slow deliberate draw of 3 - 6 seconds long to allow for adequate wicking. Think of a straw in a thick milkshake.
