I think there is a nice way to combine the solution of @chrisb and @CodeShaman (As it was pointed out CodeShamans solution counts total and not consecutive values).
df['count'] = df.groupby((df['col'] != df['col'].shift(1)).cumsum()).cumcount()+1
col count
0 B 1
1 B 2
2 A 1
3 A 2
4 A 3
5 B 1