Step 1: Understand the context of the sentence.
The sentence says there are plenty of dresses already, which suggests
that buying more is unnecessary or inadvisable, not forbidden.
Step 2: Analyse the modal verbs.
will not — simple future negation, does not fit advice.
must not — expresses strict prohibition, too strong here.
may not — expresses possibility, incorrect meaning.
should not — expresses advice or recommendation not to do something.
Step 3: Choose the most suitable modal.
Since the sentence gives advice based on the situation, should not is the correct choice.
Hence, the correct answer is
\[
\boxed{\text{should not}}
\]