This module lets you limit number of resources a user may connect.
After installing and adding the module to
modules_enabled
you can specify a limit with
max_resources
which defaults to 10.
With the plugin installer in Prosody 0.12 you can use:
sudo prosodyctl install --server=https://modules.prosody.im/rocks/ mod_c2s_limit_sessions
For earlier versions see the documentation for installing 3rd party modules