Yes - you should confirm to your guest that as he has only booked for one person, that will be the only person who will be able to check in and any additional guests could be turned away, as they haven't been booked and paid for.
Presumably you charge for extra guests - so confirm to the guest that if he does have other guests who want to stay, he must send an amendment for the correct number of people so you can prepare rooms, towels etc and so you will both be covered by Airbnb for the correct number of guests.
(he just needs to go into his booking confirmation to do this by hitting the amend button).
Unfortunately some unscrupulous guests do this to try and get away with not paying for all those who stay. It is highly unlikely one person who have booked to stay in a place that accommodates ten unless a) he intends to sneak in more guests b) he intends to have a party.
Do make sure you or your local manager is there at check in, so if extra guests try and sneak in, you can challenge them.
Personally if someone did that to me I would cancel their booking on arrival as they have lied to me, despite being given a chance to come clean. The other option is not to let the guests who haven't paid check in until the guest has gone in and amended the booking to reflect the numbers who are staying.
Make sure if you host remotely you have CCTV or similiar so you know how many people are going in and out of your property.
NB - please change your cancellation policy for flexible to at least moderate. Again, personally if I had a large property like yours, I would change it to strict.