UBound and LBound will let you determine if a given index is a valid choice within a VBScript array.
function GetNumberEight
If UBound(RowArray) > 8 Then
GetNumberEight = ""
else
GetNumberEight = RowArray(8)
end if
end function
If you need to specifically get one number from an array, though, you may want to consider refactoring your code. Eight variable declarations, or eight properties of a data object, will not run significantly slower than an eight member array.
(And if it's VB.NET, consider using the other .NET collection classes.)