The detect magic spell allows the caster to:
use [their] action to see a faint aura around any visible creature or object in the area that bears magic, and [they] learn its school of magic, if any.
The functionality of this ability is clear when an object only bears a single magical effect. However, it is less clear in the case of multiple magical effects on a single target. If the magical effects all have different schools, we can probably infer that the caster of detect magic sees a distinct aura for each school and can determine from this information that (at least) 3 spells are affecting the target.
However, suppose a target is affected by 3 different abjuration spells. Obviously the caster will see an abjuration aura on the target, but how much information will they be able to glean? Will it be obvious to the caster that there are actually three separate abjuration auras, and therefore three spells, on the target, or will they only know that there is at least 1 abjuration spell on it?