A fish like a serpae lives in groups, where they like to sort out rank. The leading fish will spend a lot of time informing the others it's still the leader. A leader with one follower ends up with a lot of pent up energy, and the follower is at a total dead end. So they try to include other fish in their hierarchy, get frustrated, and nip at them. The other fish don't follow their rules, and they have a hard time with that.
In a group of 6 to 10, they usually have no time for any other fish. They're too busy with their High School hallway drama.
They're pack animals, and need a group. Two are almost guaranteed to be nippy, but 6 or more are probably going to be feisty but okay.
Corys are also social fish, though their world is gentler. They also act happier in groups, and happy fish are more fun to keep. A lot of our small fish have a safety in numbers mindset. They adjust to being in small groups, but you don't get to see how they would act if they were comfortable in numbers. With 5, you're okay, and so are they, but 8 would make them happier. The key is the new fish don't have to be just Corys, but the same species in the Cory group as the ones you have.
I have a 120 gallon with 2 large filters. I religiously change 30% + of the water every 7-8 days, with a whiteboard close to the tank to remind me if I slip up. It's a stable, healthy tank holding 18 cardinals, 50 Pristellas, 6 pencils, and groups of Hoplisoma (Corys) melini, atropersonatus, and five different tetra species in numbers ranging from 6 to 10 each. I'm trying to breed the tetras to increase numbers.
The tank is heavily planted, and I don't slack on maintenance. I've been on that water change routine for years. It may sound like a sick way of thinking, but 50 cardinals, weight wise, are two small dwarf Cichlids. I can have large numbers because the fish are small, and have similar needs. Sometimes, they can almost vanish into the plants, and the tank only looks crowded when the food goes in.
So how many fish you can have depends on how well planted the tank is, how well filtered it is, how carefully you learn about the fish before you buy them, and most importantly, how organized you are with water changes and maintenance.
