How many LongFin Serpae Tetra should be in a school?

Serpae Tetras need a school of 8-10. This is because they are one of the most aggressive Tetras available in the aquarium hobby, and they need at least that many to spread out aggression within the school. With fewer than that, the fish lower in the pecking order can literally be bullied to death, and can also take a lot of damage to their fins from the biting that these fish are prone to. With 8 or more, the aggression is spread out so no one fish gets the brunt of it. These fish also need at least a 20 gallon aquarium, ten gallons is too small.

More peaceful types of Tetras (e.g. Ember, Flame, Glowlight, etc.) can make do with 6 because they don't have the aggression issues. Since most schooling fish are more peaceful, people give 6 as the standard schooling number, but it doesn't apply to more aggressive schooling species.

5. It's always five for schooling fish (tetras, danios, barbs, rasbora, minnows and some others).

"Community fish" like guppies, platies, some loaches, and others need at least 3 to a tank.

The more the better. A 10 gallon should be fine with 10 of them. I found that when I had serpaes in my 10 gallon they just hid all of the time in their own little territories. Now that they are in a 20 gallon they tend to school better.