Date            Number
0 2000-01-01        1
1 2000-01-02        [2, 5, 7]
2 2000-01-03        8
3 2000-01-04        [4, 11, 24]

Date            Number
0 2000-01-01        1
1 2000-01-02        7
2 2000-01-03        8
3 2000-01-04        24

df.Number = [max(i) if isinstance(i, list) else i for i in df.Number]
print (df)
         Date  Number
0  2000-01-01       1
1  2000-01-02       7
2  2000-01-03       8
3  2000-01-04      24