df_test = pd.DataFrame({'id':[1,1,1,2,2,2,2,3,4],'value':[1,2,3,1,2,3,4,1,1]})
	id	value
0	1	1
1	1	2
2	1	3
3	2	1
4	2	2
5	2	3
6	2	4
7	3	1
8	4	1

df_test.groupby(['id'])['value'].count()
id
1    3
2    4
3    1
4    1
Name: value, dtype: int64

What I want to do: select one for each id
df_test.groupby(['id'])['value'].count()
id
1    1
2    1
3    1
4    1
Name: value, dtype: int64