@Templer0
Don't cancel - too highly penalized.
You need to put your foot down. Your house, your rules.
Tell them through the ABB message page that they may in no case exceed the maximum 15 guests as is stated on the listing, and that the other 13 will have to find another place to stay.
I hope that their message was on the booking message page so that there's a record of it for ABB. Also tell them that you are happy to contact Airbnb to find other lodgings for these guests. Keep ABB phone no. on speed dial in case you need to contact them when they arrive.
See how they react. Do not negociate.
And I personally would also do an ID check (write down pasport nos.) upon arrival.
There is nothing stated in your house rules saying that only booked and registered persons are allowed on your property or the booking may be cancelled. For that reason they might sneak in the others later. Think of how you would handle that.
Many of the Community posts are about more guests coming than are allowed. You might want to peruse them for tips.
Good luck.