Handheld deathmatch!
> A Game Boy version was developed by the original developers, Xanth Software F/X, and published in 1991 by Bulletproof Software, under the title Faceball 2000. [snip] It is notable for being the only Game Boy game to support 16 simultaneous players. It was thought it did so by connecting multiple copies of the Four Player Adapter to one another so that each additional adapter added another two players up to the maximum - thus seven such adapters would have been needed for a 16 player experience.
And three months ago I had a nice discussion with one of the people responsible for the Game Gear version: