How many fish need to be in a school?

It depends on how large your aquarium is. If you have a 10 gallon its already very overstocked and I would consider upgrading very soon.

If its a 20-30 you'll have a bit more room. The neon's seem fine but corey's need 6+, Danio's need 6+, and Tetra's need 6+.

So stocking should look like this:

BS Tetra - 6

Cory - 6

Danio - 6

Neon -5

With a 20 that's a bit overstocked but okay if you have a large filter capacity. With 30g that's fully stocked. With 10 that's not possible and you should restock the entire tank.

EDIT: Guppies need groups of 1 male to 3 females or all females. They are social. I would return two of the species of schooling. (the neons are fine but the danios and tetras are not). (You ask them if they can take fish from an overcrowded aquarium, petco and petsmart most definitely do but it depends on if they are stocked. Local fish stores is sketchy too).

Then the suggested stocking would be:

5 neons OR 6 BS tetras OR 6 reddish brown danio's.

6 corey cats

4 guppies (all females or 1 male with 3 females).

You would then have a fully stocked and happy aquarium.

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I would change those 3s to 6s. 3 is about the bare minimum for these fish, 5-6 of each is preferable.

A school is six or more. What is your tank size? You might be better off keeping only three species in proper schools to avoid being over-stocked.

All of these fish are schooling fish and need to be kept in groups of 5 or more. Depending on the tank size, you might want to get more fish in each school. ( Pick me for the best answer PLEASE )

6, 6, 6, 6, 6.

Black Skirt Tetras are aggressive in small parties.

4 Black Skirt Tetras should be sufficient to decimate your tank.

Looks like you have a fair amount.