First, let's find the price of the CD after applying the coupon. The original price of the CD is $13.59, and Tony has a coupon for $1.85 off.
To find the price after the coupon:
\[ 13.59 - 1.85 = 11.74 \]
Now, we determine how much change Tony will receive after paying with a $20 bill:
\[ 20 - 11.74 = 8.26 \]
Now, we need to choose the best whole number estimate of the change:
The total change calculated is $8.26. The closest whole number to $8.26 is $8.
Therefore, the best whole number estimate of how much change Tony gets is:
\[ \boxed{8} \]