#!/usr/bin/python3
a=open('./big_file.txt', 'r')       
file_counter=0
temp_file=open(str(file_counter)+".txt", 'w')
for str_num, string in enumerate(a.readlines()):
    if str_num%500==0 and str_num!=0:
        file_counter+=1
        temp_file.close()                     
        temp_file=open(str(file_counter)+".txt", 'w')
    temp_file.write(string)
temp_file.close()

-------------------------------------------
vodka@vodka-PC:/tmp$ cat ./big_file.txt | wc -l
50000
vodka@vodka-PC:/tmp$ cat 55.txt | wc -l
500
vodka@vodka-PC:/tmp$ ll [0-9]*.txt | wc -l
100

$ split --lines=100 -d input.txt '' --additional-suffix=.txt -a 3

1. Есть split

2. Проще чем питон:
   awk -v f=1 -v n=100 '        {print > f".txt"}
                        !(NR%n) {f++            }'