Learn Swedish: Isvaken har alltid kallats isande kall. - The ice hole has always been called freezing cold.

Isande is a present participle (formed from the verb isa, meaning to freeze or chill) that is acting as a degree adverb here. It intensifies the adjective kall, creating the meaning "freezing cold". Like all present participles in Swedish, isande never changes its form, regardless of whether the noun it refers to is an en-word, an ett-word, or plural.

Does isvaken have something to do with the word vaken (awake)?

Not in this case! While vaken is indeed the Swedish adjective for "awake", here it is the definite form of the noun en vak, which means a hole in the ice (often cut for winter bathing or fishing). Combined with is (ice), isvaken simply means "the ice hole".

Why does the verb kallats end in an -s?

That -s is the passive marker. The active form har kallat would mean "has called" (for example, a person calling someone's name). By adding the -s to make har kallats, the meaning becomes passive: "has been called".

Would kall change if we were talking about multiple ice holes?