The short answer is that this is a sentence without a direct object
The verb used in a transitive passive sentence depends upon the plurality of the object but this is an intransitive sentence and the verb must always be singular.
Long answer.... Transitive sentence....
In Bologna people eat a lot of meat ( Active form) In Bologna a lot of mat is eaten (passive form)
Since the object is singular the verb must be singular.
A Bologna la gente mangia molta carne.( Active form)... A Bologna si mangia molta carne. (Passive form).
If the direct object now becomes plural...so does the verb.
A Bologna la gente mangia molti gelati..( Active form).. A Bologna si mangiano molti gelati.
However if the verb is Intransitive or there is no direct object the verb is ALWAYS singular.
In Italy people go to Rome ( Indirect object singular) = In Italia le persone vanno a Roma = In Italia si va a Roma
In Italy people go to the cities ( indirect object plural) = In Italia le persone vanno nelle città = In Italia si va nelle città.
To summarise...the verb can only be plural if the verb is transitive and has a direct plural object.
Don't forget that if the verb is reflexive...the possibility of ...Si Si..exists in this case it becomes Ci Si