Mukhalinga is a variant of the linga, a pillar-shaped item that represents Shiva.
On the shaft of a mukhalinga, one or more faces (mukha) are
carved.
The number of faces on the linga should not exceed the
number of doorways in the temple, according to texts detailing the shape and
construction of Hindu statues.
Thus, a temple with one entryway should have one mukhalinga,
and so on, up to four mukhalingas.
These faces should likewise face the entrances, according to
the guidelines.
T. A. Gopinatha Rao's Elements of Hindu Iconography,
published in 1981, is a good source of knowledge.
Discover more about Hindu Art, Architecture, and Iconography here.
You may also want to read more about Hinduism here.
Be sure to check out my writings on religion here.
