Incidentally, this might be a bit confusing, but 今年の夏/この夏最多の73地点で猛暑日になった。, i.e. the one without the comma is used for an explanation regarding a specific day in this summer, usually today or yesterday.
(今日/昨日は)今年の夏/この夏最多の73地点で猛暑日になった。
(Today/Yesterday,) 73 places had a temperature of 35°C or higher, which is the record in this summer (until now).
Thus, 今年の夏/この夏最多 means "record in this summer", but 最多 is 過去最多 "record in the history/the largest number ever" in that sentence.