you will have to go to team deathmatch and open up the options, it will show a time limit and the number of opponents you want against and some other crap.
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If your talking about multiplayer, you can't. If you are talking about offlline you just go into game setup or whatever and it should be in there..
you can do this if your doing a custom match like combat training or private match select game options and you can change the time limit
U have to sign in with a different profile with all the contollers you want to play with then you will be able to edit game options and stuff
18 every game so 18 is the limit.
Only Ascension has a round limit, 50. All others go on forever exponentially.