Do not make noise when you visit 'a patient' in the hospital.
Do not make noise when you visit 'patients' in the hospital.
Practically, these are used to mean the same thing, don't make noise when you visit any patient in the hospital, regardless of the number of patients you actually visit.
Sentence 2 does mean more than one patient, but plurals can be used as a shortcut if the speaker/writer doesn't really know if one or more than one of something will be involved, or the exact number of something doesn't really matter. The speaker/writer may be assuming that you would see 2 patients or more if needed.
The technically correct thing to say would be:
Do not make noise when you visit a patient or patients in the hospital.
but it's wordy and makes it sound like you are trying to CYA.