ID   Alphabet   Number1    Number2
1       A         NaN         9
1       A          3          5
1       A          1          4
1       A          2          4
2       B          7          3
2       B          2          8
2       B          4          1
2       B          8          5
3       C          2          2
3       C          1          9
4       D          2          3
4       D          6          2
4       D          8         NaN

df.groupby('Alphabet')['ID'].nunique()

A    1
B    1
C    1
D    1

B    1
C    1