I read a the following article.
This article mentioned how to express multiple iPhone. In the article, Phil Schiller, who is the senior Vice President of worldwide marketing at Apple Inc., tweeted, "iPhones" doesn't have to be used. For example, "two iPhone devices" and "three iPad Pro devices" are correct usage the plurals of iPhone and iPad are "two iPhone devices" and "three iPad Pro devices". Details are followings;
I searched other words can be both singuler and plural. e.g. deer, sheep, fish, cattle, salmon, moose.