Lower-than-expected demand for Apple Inc.’s new iPhones and the company’s decision to offer more models have created turmoil along its supply chain and made it harder for Apple to predict the number of components and phones it needs, people familiar with the situation said.

In recent weeks, Apple slashed production orders for all three of the iPhone models it unveiled in September, these people said, frustrating executives at Apple suppliers as well as workers who assemble the phones and their components.

Forecasts...