Fill a new pandas column with row numbers

Use numpy.arange by length of DataFrame:

df['C'] = np.arange(len(df))

Or you can use DataFrame.shape, thank you @Mehmet Burak Sayıcı:

df['C'] = np.arange(df.shape[0])

print (df)
       A   B  C
100    0   7  0
203    5   4  1
5992   0  10  2
2003   9   8  3
20    10   5  4
12     6   2  5