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Apple earnings: The one number that still reigns supreme

When Apple reports earnings after the bell Tuesday, one number remains more important to investors than the rest: iPhone sales.

"At the forefront, every time Apple reports, it's what are iPhone sales going to look like?" said Angelo Zino, senior industry analyst at S&P Global Market Intelligence. Zino told CNBC that he expects Apple to report that 51 million iPhones were sold in the quarter ending in March. That's about a 17-percent decline, year over year, he said, based on his own estimates.

"By far ... their most difficult quarter on a year-over-year basis," Zino said. "However, our view is you're going to see a less, a more modest decline in the June quarter, about a 5 percent shipment decline, and thereafter, you should start to see growth."

Zino said he expects the iPhone 7 to reinvigorate Apple's business.

The iPhone's success hinges on consumers' upgrade cycles, and there's some concern among smartphone-makers that consumers will replace their devices with newer models less frequently than before. Zino, however, remains optimistic.

Apple did not immediately respond to CNBC's request for comment.