For a portable power bank that you can easily travel with, you should look for a minimum of 10,000 mAh. Most models nowadays even offer 20,000 mAh for reasonable prices. The more mAh, the better, just be sure to weigh your capacity needs against the physical size of the power bank. Generally, the more mAh a power bank has, the larger its physical size and the heavier it is as well. Therefore, it is recommended when looking at a power bank, that you first consider the amount of power you need to charge your devices and then adjust your expectations based on the size and weight you are willing to carry around with you.