What's the maximum number of listeners an Oracle database can have?
To the best of my knowledge, there is no limit.  Of course, it is exceptionally rare that you would have more than one listener for a database.  I've seem people run two listeners just in case one listener fails but I've never heard of anyone wanting to run more than two.  If you really wanted to, though, nothing stops you from running a dozen listeners on a dozen different ports or on a dozen different machines.  But since the listener is virtually never the bottleneck and the listener virtually never fails, that sort of redundancy is rarely helpful.