The script isn't even necessary, split(1) supports the wanted feature out of the box:
split -l 75 auth.log auth.log.
The above command splits the file in chunks of 75 lines a piece, and outputs file on the form: auth.log.aa, auth.log.ab, ...
wc -l on the original file and output gives:
321 auth.log
75 auth.log.aa
75 auth.log.ab
75 auth.log.ac
75 auth.log.ad
21 auth.log.ae
642 total
