df = pd.DataFrame({ "id":[1,1,1,1,1,2,2,2,2,2,3,3,3,3,3,3,3,4,4],
     "date":("2000-07-06","2000-07-07","2000-07-08","2000-07-09","2000-07-10","2000-07-06","2000-07-10","2000-07-11","2000-07-17","2000-07-20","2000-07-06","2000-07-07","2000-07-08","2000-07-10","2000-07-15","2000-07-16","2000-07-25","2000-07-11","2000-07-20")})
df.date =pd.to_datetime(df.date)

         date     id
1   2000-07-06     1
2   2000-07-07     1
3   2000-07-08     1
4   2000-07-09     1
5   2000-07-10     1
6   2000-07-06     2
7   2000-07-10     2
8   2000-07-11     2
9   2000-07-17     2
10  2000-07-20     2
11  2000-07-06     3
12  2000-07-07     3
13  2000-07-08     3
14  2000-07-10     3
15  2000-07-15     3
16  2000-07-16     3
17  2000-07-25     3
18  2000-07-11     4
19  2000-07-20     4

count     id
1    4        1
2    1        2
3    3        3
3    0        4