The word "police" is rather special: It has no singular noun form. Something like that police over there is securing the scene would be incorrect. One would always construct sentences in the plural form like so:
The police are out in force today.
Anything done by the police will reflect on them.
Other words that take no singular form would include pants, trousers, scissors, and clothes.
Confusion arises because "police" is also used as an adjective. Consider these sentences:
A police department is housed in that building.
The police chief was highly visible at the town meeting.
In these two sentences, we are not speaking of "a police". You could easily remove the word from both sentences and they would make sense semantically and grammatically. Instead, the word describes the department or chief. It gives us context.
"Police" also has a verb form. You may encounter it like this:
The Boy Scout troop must police the area before they leave to remove any trash.
The verb means "to investigate, to search, to clean up". This certainly does fit in with a subset of the duties of a police department.
To my (American) ear, "What would most people know about insulin?" is correct, and "What would most people knows about insulin?" is incorrect.
"What would the most people know about insulin?" sounds unnatural. "The most people" is usually used as a superlative, not as a way of referring to a specific group of "most people". "Most people" is deliberately vague as to which people are in the group, so a non-superlative use of "the most people" is usually self-contradictory.
"What is the most people you have ever talked to at the same time?" does sound natural. Notice that this question asks for a superlative -- the result of comparing the sizes of all of the groups of people "you have ever talked to" -- so this use of "the most people" refers to a single group of people. "What is the largest audience you have ever spoken to?" and "What is the largest group of people you have ever talked to?" sound even more natural.
"The most people I have ever talked to was 2,500 people, during a meeting at Town Hall." In this usage, "the most people" is singular.
Best Answer
The answer isn't really about whether you're listing items or not, but about whether there is one definition or more than one definition. If all of the terms have one definition, then the singular, definition, is appropriate. If there is more than one definition, then it has to be the plural, definitions.
For example, you might say
if they all mean the same thing in that context, but
because plaintiff and defendant have different definitions.