a good many people
vs.
many people
What's the difference? Does "a good many people" mean "many people and they are good"?
Does 'a bad many people' makes sense as "many people and they are bad"?
a good many people
vs.
many people
What's the difference? Does "a good many people" mean "many people and they are good"?
Does 'a bad many people' makes sense as "many people and they are bad"?
The usage a good many is an intensifier, same as very many. Similar usages include...
1: I've got a good deal of work to do before I go home
2: They give you a good-sized mug of tea in that cafe
Note this NGram showing that a great many / a great deal have consistently been (slightly) more common than the good versions for centuries. But when the referent is an adjectival past participle as in my example #2 above, you can only use good (a great-sized mug is not idiomatic).
"A good many" is a fixed phrase meaning 'a large number' or 'a lot of'. An alternative sometimes found is 'a great many'.
So far no-one has addressed the second part of your question:
'a bad many people' makes sense as "many people and they are bad"?
No it does not. As a native English speaker I have never heard the phrase "a bad many". I can see how it might look like somehow the opposite to "a good many" but it just doesn't work like that.
Some things in English don't have opposites like you might expect. For example, you might say "I got high on drugs" but you don't say "I got low on drugs".
If you want to say "many people and they are bad"? you would just say "many bad people", for example "Many bad people looted the village after the cyclone."
But not "A bad many people looted the village after the cyclone."
In this case "bad" is the adjective and it goes next to the noun "people".
"A good many people" won't necessarily form a larger crowd than "many people", but by saying "a good many" the speaker is displaying:
"Good many" is an example of the usage "good [number]", consider:
How many people turned up to the concert?
Cor, a good fifty!
It means "about 50", but is very positive and has similar invigorating effect as the hyperbolic "50 at least!" The speaker is conveying "it was about 50 people", "I am sure no one reasonable would accuse me of understating that number", and "I am happy that it was about 50 people".
The effect of "good [quantity]" on the listener is generally that it stands out, impresses them, nudges them to share the same emotions or evaluation as the speaker.
You can also say "a good few", wherein "good" has the same positive intensifying effect. "A good few" means "a substantial few" — enough to make an impression on me even though it's admittedly "few".