No need for an Apply to each, a variable and all the stuff that makes flows complicated.
Just make use of the function range() and add a new property with a Select:

Select
From
range(0, length(outputs('Compose')))
Map
addProperty(
outputs('Compose')[item()],
'RowNo',
mul(
add(item(), 1),
10
)
)
Result:
Title,Score,RTime,RowNo
Item 0,2000,12,10
Item 1,2001,11,20
Item 2,2000,10,30
Item 3,2001,9,40
Item 4,2000,8,50
Item 5,2001,7,60
Item 5,2000,6,70