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When it comes to baseball, I don't really know much.
Literal
Baseball [with-と] say thing [to-になる][cond-と] not-much am-knowing-[not-ない][explanatory-のです].
~ということになると ('when it comes to (the matter of) X') is a fixed idiom combining quotative と + いう + ことになる ('become the case that') + conditional と. It's a set way to introduce a topic shift and signal speaker's uncertainty or limited knowledge about that topic.