com = 0 This search generally does not indicate grammatical correctness or incorrectness, but gives an idea about the frequency of the phrase. Now you know that "I suppose not" is used more than "I don't suppose so".
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With the verbs suppose, imagine and expect we can make negative forms in two ways:
subject + verb + not // subject + do not + verb + so
I suppose not. // I don't suppose so.
I imagine not. // I don't imagine so.
I expect not. // I don't expect so.
The Heinemann English Grammar