I recently bought one from a brand called "Ravpower," which is comparable to Anker. Both of those brands are great and super affordable. The one that I got claims to charge an iPhone 8 a total of 5.5 times. I haven't tested that yet, but after 2 full charges (one with an iPhone 6, one with an iPhone 7), it still shows 4/4 battery indicator lights, so I'd say they're probably pretty spot on in their claims.