How do you know if the # of stereoisomers doesn't equal 2^n

[onedirection]

The title says the question

I was sure that the number of stereoisomers is equal to 2^n, where n is the number of chiral centers

but in some questions I've taken that wasn't the case

How can we tell?

Edit; ignore, forgot about looking for meso structures

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[monkeyvokes]

The title says the question

I was sure that the number of stereoisomers is equal to 2^n, where n is the number of chiral centers

but in some questions I've taken that wasn't the case

How can we tell?

Edit; ignore, forgot about looking for meso structures

be cognizant of when a question says "how many possible stereoisomers" vs "how many"
I've fallen for that mistake in practice tests