I hear that October has the highest number of tourists every year.
Literal
Every-year October [subject-が] tourists's number [subject-が] best becomes [hearsay-そうだ].
Double が: the first marks 10月 as the noteworthy item, the second marks 数 as the inner subject of 最高になる. そうだ attached to the plain-form verb is the hearsay 'I hear that X,' 'they say X' — distinct from ~そう ('looks like X') which attaches to stems.