The amount of brightness for LED bulbs vary significantly among different manufacturers. Lumens can be counted for each LED bulb incorporated in a light strip or for the whole strip together. The ideal amount depends on the kind of activity you are going to perform below the cabinets.
For example, if you need them only for decorative or aesthetic purposes, 150-250 lumens will be enough for each foot. On the other hand, for carrying out cooking or similar tasks under these lights, you will need 175 to 550 lumens per foot.