You can't add extra for a single guest @Karelle6 , you'll have to do it the other way around:
Make the base price the price that you want to charge for single occupancy.
Then add the "extra guest fee" from the second guest (it's in the Pricing tab, scroll all the way down to Additional Charges). It sounds like you want the extra guest fee to be smaller, so that you earn relatively more per guest if it's just one guest.
You should be able to get an idea of what other hosts in your area charge if you do a map search for similar properties and change the number of guests from 1 to 2 to see how the price moves.