Values
 

From highest value in A1 to A5, I want a 10 then 8 etc inserted automatically
in B1 to B5

=INDEX({10,8,6,4,2},RANK(A2,$A$1:$A$5,FALSE)+COUNT IF($A$1:A2,A2)-1)

if you want different numbers for equal values in A1:A5, otherwise:

=INDEX({10,8,6,4,2},RANK(A2,$A$1:$A$5,FALSE))

HTH,
Bernd