"Caliente" is used for things, including beverages. "Caluroso" is for weather. There's more, of course. If you're interested:
I said "In summer there are many hot days." This is how that would be said in English. (Native speaker NYC) It would not be "summer has a lot of hot days." "Summer" does not have things in English, unless maybe you named your daughter Summer. Why do they want a literal translation instead of a correct one? Isn't the point of language instruction to recognize that literal translations are often incorrect?
Fellow American here, and "The summer has many hot days" doesn't strike me as an unusual English sentence at all.
Duolingo has to strike a balance between literal correctness and idiomatic correctness, and it can't possibly account for all plausible translations. But if you think yours should be included, always use the report button to say so.