This is the content of testing directory:

user@host /tmp/temp % ls
file.txt

Split one file into 30 lines containing files:

user@host % split -l 30 file.txt

Result:

user@host % ll
total 28
-rw-r--r-- 1 user user 2361 Dec 12 10:05 file.txt
-rw-r--r-- 1 user user  457 Dec 12 10:06 xaa
-rw-r--r-- 1 user user  468 Dec 12 10:06 xab
-rw-r--r-- 1 user user  431 Dec 12 10:06 xac
-rw-r--r-- 1 user user  498 Dec 12 10:06 xad
-rw-r--r-- 1 user user  485 Dec 12 10:06 xae
-rw-r--r-- 1 user user   22 Dec 12 10:06 xaf

Verification:

user@host /tmp/temp % wc -l xaa
30 xaa

Counting lines in all created files:

user@host% wc -l x* 
  30 xaa
  30 xab
  30 xac
  30 xad
  30 xae
   2 xaf
 152 total

Final command, with defining additional suffix .txt and output prefix file:

split -l 30 -d --additional-suffix=.txt file.txt file