Grouping is the core trick
The visual system does not seem to keep a separate tally of every object in a busy scene. Instead it forms ensembles: sets of items perceived as a unit. In a tracking experiment, people could follow about four groups of moving objects, and that limit held regardless of how many items made up each group [2]. That is a striking result: four groups of two and four groups of six are equally hard, which suggests the bottleneck is the number of groups, not the number of things.
Hearing works the same way in principle. Overlapping sounds are grouped or separated so that we can identify distinct sources, a process called auditory scene analysis. Cues such as harmonicity (whether tones share a common pitch pattern), timing, frequency, and spatial location all push sounds toward being heard as one source or several [1]. The system is not measuring each sound wave; it is deciding which waves belong together.
What holds a group together
For moving objects, a shared fate matters. When objects move together rigidly, they read as a group; when that common motion is disrupted, tracking performance drops [2]. Spacing matters too: as the distance between objects grows, group tracking gets worse [2]. Both findings fit the idea that the brain uses simple heuristics, like "these things move as one" or "these things are close," to decide what counts as a single item.
The auditory cues play an analogous role. Sounds that start and stop together, share pitch structure, or come from the same direction tend to be fused into one source, while mismatches on those cues promote separation [1]. In both senses, the grouping rules are the same kind of shortcut: use whatever regularities are available to guess which pieces belong to the same thing.
Attention sets the ceiling
Even with good grouping, there is a limit. Attentive tracking of individual items tops out at roughly three to five items [2]. Grouping does not remove the limit; it changes the unit being counted, so the ceiling applies to groups instead of items [2].
Attention and prior knowledge also shape what listeners actually hear. The auditory system's organization of a scene is described as a best guess that integrates multiple cues together with cognitive factors, which is why the same mixture of sounds can be heard differently depending on what you are trying to follow [1]. The precise neural basis for this organization is still under investigation [1].
Machines face the same problem
The challenge is not unique to biology. In autonomous driving, a system must simultaneously detect 2D objects in camera perspective and 3D objects in a bird's-eye view from multiple cameras, and one approach couples the two detection tasks so each informs the other in a repeating refinement loop [3]. That is an engineering solution, not a description of human perception, but it illustrates the same underlying difficulty: deciding what is out there when many signals arrive at once.
A broader ecological view holds that perception is primary and that we cannot perceive the "true" shape or color of things, only what is available to us [4]. Read alongside the grouping evidence, that suggests our experience of many things at once is a constructed, good-enough summary rather than a complete inventory [1][2][4].
Sources used in this answer
Perceiving Multiple Sounds
The auditory system groups or segregates overlapping sounds into distinct sources using cues such as harmonicity, timing, frequency, and spatial location, with attention and prior knowledge shaping the result, and perception described as a best guess whose neural basis remains under investigation.
Ensemble perception during multiple-object tracking
People track about three to five individual items but about four groups of objects regardless of group size, and group tracking worsens with greater inter-object spacing and with disruptions to common motion.
SimPB++: Simultaneously Detecting 2D and 3D Objects from Multiple Cameras
SimPB++ is an end-to-end model that simultaneously detects 2D perspective objects and 3D bird's-eye-view objects from multiple cameras, coupling the two tasks through interactive refinement.
The primacy of perceiving: An ecological reformulation of perception for understanding memory
An ecological reformulation argues that perception is primary and that one cannot perceive the true shape or true color of things.
Electrophysiological correlates of multiple object processing
Processing multiple objects simultaneously, as when walking in a crowded street, is described as relying on at least two mechanisms tied to attention and visual working memory.