IRC network
An IRC network is technically a network of one or more IRC servers linked together[1] that allows people to chat together on it independently from the chosen server.[2]
More than 50.000 simultaneously connected people may be found on some of the biggest networks[3][4] and there are more than 700 networks known to the IRC search engine netsplit[5] and assumingly many others not known to the search engine.
IRC networks are often run by a loose group of people and, in case of bigger networks, also by foundations or organizations which have sometimes been created just for the purpose of running and funding an IRC Network (e.g. Freenode).[Source?]
What defines an IRC network
A network has often a specific theme or a specific purpose. It might be a place to meet for people of a specific country, a specific age or a specific group, e.g. gamers or IT freaks. Today's small networks sometimes even consist of one server, but are an own, seperated themed area to other networks.
The attributes of any long-term growing network that exceeds a short-time available fun or testing server are an own user subculture and availability close to 100%. If one of those two attributes is not met, it is unlikely to ever grow out of the 10 user stage.
Technical side of networks
Bigger networks are, as the word already implies, a union of multiple servers that work together. The benefit of this is a much better reliability of the network as a whole. When a server drops out, others still offer a place to chat and apart from the affected users on the server that dropped out (or in case of net split that causes a bigger disruption to users), other users can simply continue chatting. Affected users would simply connect to another server which can be done in a few seconds.
Basically, a network would only rarely need to grow more than one server with todays hardware, but separating the users over different machines just lowers the chance of an overall dropout which can cost a network many users.
Apart from that, other users can more easily contribute as admins to the network and have more ways to change the network and to participate. Servers spread across the world can also offer a better connectivity to users in a specific country than a server that is half around the globe.