Traditionally thought to be 7 things, more recent studies suggest 4.
Working memory has been thought to hold ~5 to 9 things. This “Magic of the Number 7” extends to several other things. People can generally hold about seven different “bucketing groups” in their working memory for things like “pitch frequency” (I’ll play you a tone, you tell me which number it corresponds with, we start to fail when the number of tones goes above 7), or 7 levels of taste for things like saltiness or sweetness.
Famously, it’s why phone numbers have 7 digits.
It’s also why, in the realm of Systems Engineering, we usually decompose systems into whatever number of levels is needed such that no level contains (much) more than 7 parts. (see Systems Decomposition Rule of 7)
According to The Organized Mind, more recent studies suggest the number should be more like “4”.