"Many stars can be seen" is a 'passive' translation, and I agree that the plural should be used, because 'stars' is plural.
The English sentence "One can see many stars" is written using the 'active voice'. There is a subject. The 'passive' lacks a subject.
When the subject in the active voice doesn't designate a specific person, it's translated into Spanish using the 'impersonal 'se'' construct. The singular form of the third-person ending is used.
Native speakers sometimes confuse these two constructs.
