A listener can serve various instances,

LISTENER =
(ADDRESS_LIST =
(ADDRESS= (PROTOCOL= IPC)(KEY=orcl))
(ADDRESS= (PROTOCOL= IPC)(KEY= PNPKEY))
(ADDRESS= (PROTOCOL= TCP)(Host= kerrigan)(Port= 1521))
)
SID_LIST_LISTENER =
(SID_LIST =
(SID_DESC =
(ORACLE_HOME= /oracle)
(SID_NAME = orcl)
)
(SID_DESC =
(ORACLE_HOME = /oracle)
(PROGRAM = extproc)
)
(SID_DESC =
(ORACLE_HOME = /oracle)
(SID_NAME = exp)
)
)

for instance this listener.ora is serving for 3 instances