(Note: I have edited this answer to remove extraneous opinions)
"When having symptoms of fever" is not quite idiomatic, making me think this sign appears in a non-English-speaking country. It's grammatical, but I feel like there are more idiomatic expressions.
Aside from that, though, no one would read this sign and expect to have to share a mask with someone else. The natural implication is one mask, each. Example:
All applicants should remember to sign their form before they turn it in.
This helps emphasize there is a single form, which each applicant must sign.
That being said, it might be more idiomatic if the sign in your example said:
Passengers are advised to wear surgical masks if they have signs of fever.
Again, no one would think each passenger was supposed to wear multiple masks. It just matches the plural "passengers" with plural "masks".
Most of these kind of regulations are written in the imperative form: "do this", "don't do that", etc. and many seem to use the plural. For example NCTD Rules of Riding:
Attire: Shirts and shoes required at all times.
Luggage, Surfboards and Other Belongings: Passengers’ belongings, including surfboards, must not exceed 6 feet in length, and must not block seats, aisles, doorways or exits. Surfboards and open strollers are permitted only on the lower-level of cars.
It's unlikely anyone would be carrying more than one surfboard or stroller, but using the plural allows for any combination, while using the singular creates an expectation of one, each.