1. Which of the patients have recovered?
2. Which of the patients has recovered?
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Do we have to use 'has' or 'have'? Are both OK? Which one is common?
Both can be correct, depending on how many recovered. 1 implies that more than one recovered. 2 implies that one recovered.
which one has recoverd
Both "has" and "have" can be used to construct sentences, but their usage depends on the subject of the sentence.
In the case of your two questions, the subject is "patients." The word "patients" is plural, which means that the correct verb form to use with it is "have." Therefore, the correct sentence would be:
1. Which of the patients have recovered?
If the subject was singular, we would use "has" instead:
2. Which of the patient has recovered?
Using the correct verb form is important for subject-verb agreement and to ensure that the sentence is grammatically correct. Although both "has" and "have" can be used, "have" is more commonly used with plural subjects like "patients."