For a harry potter neighborhood I was gonna do my own NPCs and townies which would works since we don't family trees for many HP characters/students , now how many should I do in total for mainhood + shopping district + downtown + college

here's some I already had in mind

game's character limit for any neighborhood is 7fff in hexadecimal; 32767 in decimal.
not entirely sure, but I imagine that is much more than any computer could handle.

any number less than what your computer could handle would work fine.

If you use the townie generator, it will spawn 30, representing all ages. (Not much help, I know ) That would be in the main hood.

When I created townies for my current neighbouhood I created 10 male/female adult townies, 5 male/female teenager townies, 0 child/elder townies and 3 of each NPC.

It worked for me, but I hardly ever even acknowledge the presence of Townies, so...

Just curious, why did you choose 3 of each NPC? Did you want to prevent them being generated?

However many you want. I only ever make a few townies and gradually either age them up or move them in to make playable and create a few.

I think it's really just up to you and your play-style. I like to play with sim owned businesses and I find I need a good group of townies for that. I tend to make around 20 or so to start with and then I add as feel the need. I make a minimum of 15 dormies and slowly add more to get up around 40. However I often will age up townie teens to become dormies and then make more teens to take their place so it's not all me just making new dormies. I then often will age up dormies that have hung around for a gen or two and make them townies. It's just my way of keeping the population refreshed and so I don't have adults who find their teen dating the same townie teen they did.
I let the game generate NPC's as I need them and I only have one of each, NPC's are just worker bees to me and my sims interact with them only in their work related capacity. I did learn in the first hood I made using empty templates not to marry playables to each other for the first couple of generations or your gene pool gets really small really quick. Especially since I also use a mod for extended families so Sims recognize their 1st and 2nd cousins as being family.

Nah I just wanted variety and be able to choose what they look like :-)

Would have been boring if every house would have had the same gardener/maid.