Despite there being 6 Kingdom Hearts games, only 2 have ever appeared on the same system in their first edition.
Since the original Kingdom Hearts game was released in 2002, five more have surfaced, each on a different system. The original Kingdom Hearts was released on the Playstation 2. The next was Kingdom Hearts: Chain of Memories, which was released in 2004 on the Game Boy Advance.
Kingdom Hearts II was released in 2005 on the Playstation 2, and is the only other one of the series to be released on that platform aside from the original. In 2008, Kingdom Hearts Coded was released on mobile phones. In 2009, Kingdom Hearts 358/2 Days was released to the Nintendo DS handheld console.
In 2010, the most recent game was released, which was Kingdom Hearts Birth by Sleep, on the Playstation Portable. That means that there have been a total of five systems that the games have appeared on, with only Kingdom Hearts I and II sharing a system.
If second editions are included, such as Kingdom Hearts II: Final Mix+ (PS2, 2007) and Kingdom Hearts: Re: Coded (DS, 2011), this number changes to a total of four or five games on the same system (as Kingdom Hearts II: Final Mix+ contained Chain of Memories as well), and is somewhat less impressive.
What's more interesting is that this trend is going to continue. This year, Kingdom Hearts: Dream Drop Distance will be released to the Nintendo 3DS, which will bump the platform number to six.
Furthermore, Kingdom Hearts 3 will most likely be released on the successor to the Playstation 3, which will bump the number to seven.