Assuming odd one digit numbers are to be positive non-zero numbers, namely 1,3,5,7,9.
The possible sums are
1+1=2
1+3=3+1=4
1+5=5+1=3+3=6
1+7=7+1=3+5=5+3=8
1+9=9+1=3+7=7+3=5+5=10
3+9=9+3=5+7=7+5=12
5+9=9+5=7+7=14
7+9=9+7=16
9+9=18
I count 9 numbers.
Zero is considered even.
If one digit numbers include negative numbers, proceed similarly as above.
