Looking into the code (by searching for the error message in the github repo) I got to this file:
It seems that there is no possibility to set the “maximum number of rooms per guest” value, rather the kind of license your server uses determines it.
Where can I find information on which license allows how many rooms per guest?
It may be that adding a new role which does the same as ‘guest’ but with a different name, might allow users that can not see everything but should be allowed to peak into specific channels of your rockechat community. Adding a new role also requires a non-free license, though.