A Tuesday earnings call from a mall operator included its CEO's analysis on the kinds of brick-and-mortar stores the company expected to see in the marketplace, and it included a staggering guess for one upstart brick-and-mortar retailer: up to 400 Amazon Books locations.

The Wall Street Journal was first to report on the earnings call, during which General Growth Properties, Inc. CEO Sandeep Mathrani made the estimate while fielding a question about his properties' foot traffic in an online-shopping world. Mathrani pointed out high return rates at physical stores for online-ordered products, which he estimated at 38 percent, and that fed into his follow-up estimate: "You go to Amazon opening brick-and-mortar book stores, and their goal is to open, as I understand, 300 to 400 bookstores." (Currently, Amazon only operates a single shop at Seattle's University Village shopping center.)

He added that "the last mile is all important" in terms of engaging with customers, noting that other major online retailers like Bonobos, Birchbox, and Warby Parker have plans for their own brick-and-mortar expansions throughout the United States. "It’s a very interesting evolution, because the cost of the last mile is that important," Mathrani said to investors. "The mall business, if you appreciate that it's more focused on fashion, is very different than a staple business where you’re buying commodity. In the mall business, the impact of eCommerce is a lot less—it’s actually your friend, not your enemy."

Mathrani didn't attach a timeframe to his guess about Amazon's possible book store expansion plans. Amazon representatives declined to answer our questions, particularly whether Mathrani was in a position to make such estimates, beyond telling Ars that "we don't comment on rumors and speculation."

The news follows a Guardian report in January that quoted a major bookseller in Seattle, the home of Amazon's first brick-and-mortar bookstore, as noticing "different spending patterns" since the online giant opened its first offline shop in November. The manager of University Book Store, one of Seattle's longest-running independent bookstores, went so far as to say "the sky is falling" in terms of Amazon's impact on the region's brick-and-mortar book retail ecosystem.