df['C2'] = df.groupby(['A','B'])['B'].transform('count')
df
A B C2
0 7 9 2
1 7 2 2
2 1 9 3
3 4 8 1
4 9 1 1
5 6 4 1
6 7 2 2
7 7 9 2
8 1 9 3
9 1 9 3
用于解决方案的数据
data={'A': {0: 7, 1: 7, 2: 1, 3: 4, 4: 9, 5: 6, 6: 7, 7: 7, 8: 1, 9: 1},
'B': {0: 9, 1: 2, 2: 9, 3: 8, 4: 1, 5: 4, 6: 2, 7: 9, 8: 9, 9: 9}}
df=pd.DataFrame(data)
df