There are two chiral centers in the ethambutol molecule, but only three possible stereoisomers (structures attached below).
In general, the number of stereoisomers of a chiral molecule is 2ⁿ, where n is the number of chiral centers, so for ethambutol it should be 2² = 4. However, if the molecule is symmetrical (in this case it has a plane of symmetry), then there is an R,S-isomer that is not optically active as a whole because the chiral centers cancel each other out. This is a so-called meso-isomer. So, in the end, we have two enantiomers (R,R- and S,S-) and a meso-isomer, giving us three stereoisomers in total.
You can learn more about chiral centers here:
brainly.com/question/9522537
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