The number of lines in a column in a sefer torah can vary although nowadays virtually all scrolls will be written to the format of the K'tav Tikkun which gives 42 lines. However the Rambam (Maimonides) own scroll was 51 lines and sifrey torah can be 60, 72 or 98 lines deep. The various explanations for the number of lines are as follows:
42 lines corresponding to the stops made by the Israelites journey between Sinai to the plains of Moab (Num. Chapter 33). 
60 lines corresponding to sixty myriads of Israelites classified as being of military age (Num.1:46)
72 lines corresponding to the number of elders (Num. 11:16) even though there are actually only 70!
98 lines corresponding to the number of curses and warnings (Deut. 27:28)
but even if the scribe differed from these it is not pasul (invalid). However a scribe must adhere to the rulings as regards petuchot (open) and segurot (closed) sections and any difference here does invalidate the work.