If the Acc_No's really are numbers then empty cells should not have any effect on the formula:

Data Range
A
B
C
D
E
2
Acc_No
Week 1
Week 2
-----
Count
3
700
0
1
3
4
700
1
1
5
700
1
3
6
701
1
2
7
701
1
2
8
703
0
0
9
7003
1
1
10
2
0
11


This array formula entered in E3:

=SUM(IF(FREQUENCY(IF(B3:B100>0,A3:A100),A3:A100),1))

** array formulas need to be entered using the key
combination of CTRL,SHIFT,ENTER (not just ENTER).
Hold down both the CTRL key and the SHIFT key
then hit ENTER.