As other answers mentioned, Docker's default local bridge
network only supports 30 different networks (each one of them uniquely identifiable by their name). If you are not using them, then docker network prune
will do the trick.
However, you might be interested in establishing more than 30 containers, each with their own network. Were you interested in doing so then you would need to define an overlay
network. This is a bit more tricky but extremely well documented here.
EDIT (May 2020): Link has become unavailable, going through the docs there's not an exact replacement, but I would recommend starting from here.