For most 'onboard' solutions (you plug the speakers in near the keyboard/mouse/USB jacks), there is a pretty sharp dropoff in performance and quality going from 16 simultaneous sounds to 32 - so you don't want to use 32 if you can avoid it (and they can go no higher than 32 at all - so if it lets you, and you set this option higher, you will likely break something).
Creative's SoundBlaster 'Audigy'-series cards can do up to 64, and in hardware, so it has (virtually) no performance impact at all (except the 'Audigy SE'). The X-Fi cards can do 128, again, in hardware (except the 'XtremeAudio' model).
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