[...] the total number will be more than 15, including team deathmatch, GTA race, cops 'n robbers, and hangman's noose. Among those modes are options for traditional adversarial games (deathmatch or team deathmatch, for example), or co-op games such as cops 'n crooks and hangman's noose, where teamwork is essential.

GTA IV supports up to 16 players (on both PS3 and Xbox 360) online, and each mode offers enormous customizations specific to each mode. In team deathmatch, you can set up an equally balanced eight-on-eight battle or a lopsided two-on-14 fight. You can pick a custom or quick game, and party leaders can tweak options to alter each game significantly: You can change weapon types (all handguns for instance or only snipers), add or subtract police, traffic, pedestrians, or blips (i.e., enemy location).
[...]
"If players want they can jump online and never even play the single-player again," Jeronimo Barrera, vice president of game development, told GameTap. "Online is a serious time sucker. You're going to have to be really careful if you’re married," he added with a smile.