You didn't state whether the two odd numbers to be added must be 
different odd numbers or whether they can be the same odd number.
So I'll do both cases.


Case 1: They can be the same odd number:
They are all even numbers starting with 1+1=2 and ending up with 9+9=18.
So you can get these 9 even numbers: 2,4,6,8,10,12,14,16,18.

Case 2: They must be different odd numbers:
If you require that the two single-digit odd numbers be DIFFERENT 
odd numbers, then you can't get 2 or 18, so you can only get these 
7 even numbers 4,6,8,10,12,14,16.

So the answer is either 9 or 7, depoending on whether the two odd numbers
you are adding can or can't be the same odd number.

Edwin