20 million sold so far in two months and one reported spontaneous fire in an iPhone 7 in Australia. The normal failure rate of Lithium Ion batteries is 1 in 8,000,000 to 10,000,000 per year. There are currently slightly over one billion iOS devices in the wild activated. That would translate to approximately 100 to 120 random battery failures in a normal year. That's what is being seen on iPhones and essentially other smartphones as well.
The Samsung Galaxy Note 7 was on the market for a month and world wide had shipped just 2.5 million phones, yet over 250 batteries failed, and in the US alone 94 burned and/or exploded, and 26 resulted in injuries with 53 resulting in property damage. Extending the calculation, those Samsung batteries were failing at ~3,000 times the expected normal rate for Lithium Ion batteries. Something was terribly wrong.