Enter formula in B2 and copy down
Formula: copy to clipboard
=IFERROR(1/(1/IF(A2="",COUNTA(A$2:A2)-SUM(B$1:B1),"")),"")

v A B
1
2 alkdjs
3 sdlfkjas
4 asldfkj
5 adfljk
6 4
7 asdlfkja
8 asdldsfj
9 2
10 asdlfja
11 dlkj
12 sdkjflkjadsf
13 asdlkj
14 adlsfjk
15 5