Each individual sheet of k’laf that is sewed together to form the whole of the Sefer Torah is called a yeriah. In the first instance a yeriah must contain at least 3 amudim (columns) of text though many can contain as many as 8. The number of sheets joined together varies depending on the size of the k’laf but on average this will be about 50.  For more on the parchment itself see the section on k’laf.

A sheet of torah