grammatical number - What is the plural form of trademarked product names, specifically of the term "WordPress"?

The term WordPress is a trademark; as such, I view it not as a noun but rather as an adjective, and therefore neither singular nor plural. It is a proper name that describes a software script.

The use of the term as an adjective also applies to the example: "I have two WordPress installations," or, "my server runs two WordPress instances."

The use of the term as an adjective also fits well with the cola analogy, and the trademarks Pepsi and Coke: "I would like two Pepsi colas, please," or, "I would like to Coke drinks, please." In these cases, Pepsi and Coke are adjectives that describe a particular beverage.

In the case of Coke, Pepsi, and Toyota, the trademark is being commoditized or genericized, such that the trademark becomes synonymous with a good or service. (For example: we all use kleenex rather than facial tissue; residents of Atlanta order cokes, not sodas; and Brittish housekeepers hoover, rather than vacuum, the carpet.)

In such cases, the trademark becomes a synonym of the noun that it formerly described (or the verb that defines the action performed by that noun, such as hoovering) - usually accompanied by referring to the term in lowercase, rather than capitalized (Kleenex/kleenex, Coke/coke, Hoover/hoover).

As popular as WordPress is, it has not reached the point of being commoditized; as such, I would continue to treat it as an adjective.

That said, a non-trivial segment of users - likely, comprised of the same people who refer to having a facebook, or a twitter, or a tumblr - view WordPress as a noun, and refer to having a wordpress. In such cases, I would consider the wordpress to be the hosted, served, running script; i.e. not merely the software script itself, but the resultant application comprised of the WordPress software script, the LAMP stack, the server hosting it all, and the browser rendering the output. At that point, I would consider that application to be commoditized - similarly with Facebook, Twitter, Tumblr, etc. - to the point of being synonymous with the application itself. Thus, in such cases, I would consider "WordPress" to be a noun, for which the proper plural would be WordPresses.

And as someone who still views WordPress as the trademarked name of a software script, that makes me cringe even more than hearing someone talk about their "facebook" or their "twitter".