First answer, yes. You can only have one mirroring endpoint per instance.
Regarding the second question, from MSDN:
LISTENER_IP = ALL | (4-part-ip ) | ( "ip_address_v6" )
Specifies the IP address that the endpoint will listen on. The default is ALL. This means that the listener will accept a connection on any valid IP address.
If you configure database mirroring with an IP address instead of a fully-qualified domain name (ALTER DATABASE SET PARTNER = partner_IP_address or ALTER DATABASE SET WITNESS = witness_IP_address), you have to specify LISTENER_IP =IP_address instead of LISTENER_IP=ALL when you create mirroring endpoints.