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."