What is the function of 一次 in "去一次"?
Grammatically, 次 is a verbal classifier (a classifier proper), which expresses how many times the action indicated by the verb occurs. The origin of your confusion comes because classifiers are not used in English but they are natural in Chinese.
Classifiers (verbal or nominal) must come with a numeral phrase. In this sense, the numeral 一 (one) might be interpreted here having more a grammatical function than a semantic function (i.e., it doesn't carry enough meaning to be translated in the target language). Indeed, it is possible to omit the entire numeral phrase 一次 and just say 明年夏天我打算去, provided the place is clear from the context.
As an example, the sentence 我昨天买了一本书 presents a slight ambiguity, right? It could be &quot;Yesterday I bought a book&quot; (&quot;one&quot; has grammatical function, translated as indefinite article) or &quot;Yesterday I bought one book&quot; (&quot;one&quot; has semantic function, translated as &quot;one&quot;). Though without further context, you should favor the former, because of what I said above.
Finally, whereas the number &quot;one&quot; may or may not be semantically relevant, any other numeral is likely to be so.
As always, check the context:

老朱呢？我来叫他一声 = Where's Lao Zhu at? Let me call him (&quot;one&quot; is grammatical)
我叫了他两声，他还不听 = I called him twice, he still doesn't listen (&quot;two&quot; is semantic)
那本书我不太记得，我只看了一遍 = I don't quite remember that book, I only read it once (&quot;one&quot; is semantic)
请你来帮我一下忙 = please give me a hand (&quot;one&quot; is grammatical)
我被他骂了一堆 = He scolded me badly (literally, a pile) (verbal measure word (!), &quot;one&quot; is grammatical)