Just purchased 6.5 hp Lawnboy. Do I add the whole container of oil (20 oz) that was included?

It is likely that there is already some oil in it. Check your owner manual to confirm the correct amount that the oil reservoir should hold.

Look at the dipstick that is attached to the cap to see the correct "full" amount.

Make SURE to not overfill it.

If it came in a box , and you took it out you will have to put oil in it.
If you bought it already setup , Rickj is right check the oil first.
Luck