Wwise offers a "Limit sound instances" option in the Advanced Settings tab of Wwise objects such as sounds, containers and music segments. It allows you to specify the maximum number of instances of a particular sound that can play simultaneously on a single game object in the game. This setting can be platform-specific (it is "unlinkable"), and is inherited/overridable in the hierarchy of Wwise objects.
For example, setting an instances limit of 2 on a footstep sound would mean that no more than 2 footstep sounds will be played simultaneously for each character, even if they are running very fast and more than 2 sounds would normally be heard at the same time. Which sounds will survive and which will be killed depends on the sound's priority, which can also be set in the Advanced Settings tab, and can optionally depend on distance from the listener (or the closest listener when multiple listeners can hear these sounds). When the sound engine has to decide between 2 sounds that have the exact same priority, it uses the “When limit is reached and priority is equal” option set in the Playback Limit section of the Advanced Settings to decide if the newest or oldest instance should be killed.
This works fine when you want to limit sounds on a per game object basis, but you may also want to limit the total number of footstep sounds you can hear in the game, regardless of which game object they are playing on. For example if you have a thousand soldiers walking, you probably don't want to hear 2000 footstep sounds simultaneously (1000 soldiers X a maximum of 2 sounds per soldier).
This is where instance limits on Busses come in. In the Advanced Settings tab of Busses you also have a "Limit sound instances" option. It can also be platform-specific, and is inherited/overridable in the busses hierarchy. The difference here is that this setting will limit the number of sounds that will play through this bus simultaneously. It is not limited to a particular game object, and not even to a particular sound – if multiple sounds/containers are routed through the same bus, they will all “fight” for a position within the limited number of sounds that can play through this bus.
If we go back to the soldiers footsteps example, you could create a bus named "Footsteps", set an instances limit of "20" (or whatever number makes sense -- and remember you can set this per-platform if desired), and route the footstep sounds through this bus. You will still get the limit of 2 sound instances per game object, but now the total number of instances, for all game objects (i.e. for all soldiers) will be 20 -- you will never hear more than 20 footstep sounds simultaneously, even if you have thousands of soldiers in the game.
Note that this system does not discriminate between audible voices and virtual voices: If a virtual voice has a higher priority than an audible voice, the audible voice will get killed first.
