Fill a new pandas column with row numbers

By using reset_index

df['C'] = df.reset_index().index
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

---

To generalise:

df['C'] = df.index if df.index.is_monotonic_increasing else range(len(df))
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

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