Some simple rules will keep you from being confounded when you’re trying to figure out how to make proper nouns such as “iPhone 6,” “iPhone 6S,” and “Blackberry” plural.
Product Names Are Usually Trademarks
Product names are usually trademarked, and companies don’t like you to use trademarked words generically. Making names plural counts as using them generically. The Apple page Guidelines for Using Trademarks and Copyrights gives this example:
Rules for Proper Use of Apple Trademarks
1. Trademarks are adjectives used to modify nouns; the noun is the generic name of a product or service.
2. As adjectives, trademarks may not be used in the plural or possessive form.
Correct: I bought two Macintosh computers.
Not Correct: I bought two Macintoshes.
In other words, Apple has always said it wants you to say things like “I have two iPad Pro devices.”
But we all know that’s not realistic in casual writing and speech. In fact, commenters on the Mac Rumors article helpfully provided examples of Apple executives breaking the rule in their tweets and the company breaking the rule in its press releases.
How to Make Product Names That End in Numbers Plural
In the past, listeners have asked about how to make Apple product names that end with numbers plural. For example, how would you make iPhone 6 plural?
I had addressed plurals of product names before (the recognized plural of “BlackBerry” is “BlackBerrys,” even though RIM wants you to call them “two BlackBerry smartphones”), but I’d never faced a question about a product name that ends with a version number.
It’s not terribly uncommon for a product name to end in a number--we have the Boeing 737, Heinz 57 (officially Heinz 57 Sauce), the Commodore 64, Pepsi One, and so on—but it does make you stop and think when you have to make them plural.
Common Use: iPhone 6s?
If you ignore the trademark issue, there are still varying answers to the question.
The Associated Press, in its more recent Q&A answer that comes close to addressing this question, hews to the company line, suggesting its writers write about iPhone 6s phones and iPhone 6 models.
On the other hand, I don’t think adding a number to the name should make a difference in the way you’d handle it. The number is part of the official name, so if in common, non-trademark hewing language, we’d write that we have two Blackberrys, two iPads, and two 727s, then we should be able to write that we have two iPhone 6s.