How many depends on the size of the tank and what else is in it.
As for aggression... they are not aggressive really, but they are predatory, so if it fits into their mouths expect it to be food. They are a typical catfish in that way. They are a schooling fish, so a group of 3 - 5 minimum would help keep them active and healthy. Less than 3 of them and you'd be lucky to see them at night when they came out for food and they would stress much easier/quicker.
I wouldn't keep them with anything small like neons or guppies, but in a tank with gouramis (standard), kribensis, some of the larger barbs such as the neon rosy barbs, larger tetras such as congo tetras, larger rainbows such as turquoise, etc. they would make for a fun and interesting mix. I would not mix them with dwarf cichlids, large aggressive and/or predatory cichlids such as convicts, oscars, etc. or African cichlids... and I would not mix them with other catfish (including corys), loaches, or small bottom feeding animals such as dwarf frogs, shrimp, etc.
These guys need plenty of territory to call their own, caves and log overhangs will be their favorite spots. If given the proper environment and large enough tank they will spend a lot of time chasing each other around, stalking anything they perceive as prey, and poking around on the bottom of the tank. They stress easily, I would not list them as one of the heartiest of the catfishes, but if proper care is taken to transport and acclimate them, they settle in rather quickly.
I hope this helps. If you have further questions please ask.