Last year, the iPhone 6 and 6 Plus sold 10 million units in their opening weekend, though that number excludes sales in China, where around 3 million units were sold in their first weekend.
All together, sales remain roughly the same year-over-year, though some analysts view it as a good sign.
"I'm optimistic," said Jeff Fidacaro, a senior technology analyst at Monness Crespi Hardt & Co. in New York. "13 million should be a good enough number to get the stock moving."
Investors weren't quite so sure. Apple's stock opened lower on Monday after the weekend sales were announced.
iPhones are the most important source of revenue for the technology giant, accounting for 60% of the company's revenues. Wall Street analysts sometimes refer to Apple as simply "an iPhone company," reflecting this concentration of revenues.
Apple also announced that the 6s and 6s Plus will roll out in more than 40 new countries starting on October 9, including Russia, Ireland, Sweden and several Middle Eastern countries.
While the blockbuster sales are encouraging for Apple, the iPhone still accounts for only 14.6% of the global cell phone market, according to a study by Gartner, so there's still a lot of room for sales to grow.
"The question for Apple is whether there's an opportunity to grow their customer base and expand market share," said Gene Munster of Piper Jaffrey, "or if the iPhone is simply a phone for rich people."