I would give them a call right away and ask how many people are occupying the unit. If they say six, mention that you are checking because your outdoor security camera recorded 13 people entering the unit with luggage. See if they come clean at that point. Then let them know that, in accordance with the contract they signed, you will be billing them for the extra people. If they do damage to the unit, bill them for that too. Going forward, I would definitely recommend charging a refundable security deposit. In the contract, say the deposit will not be refunded if there is damage to the unit or if the number of occupants exceeds the agreed upon number.
My parents own a rental home in South Lake Tahoe. With two bedrooms, a sleeping loft and only one bathroom, they limit the occupancy to eight. There is a paved driveway next to the house which can only fit two cars. The instructions say that only two cars can be parked and there is absolutely no on-street parking. So a few years ago, I started talking to a mom at my kids' school about Tahoe. She said her family rented the same place every summer. They would take two other families and set up air mattresses all over the floor. She said they could get up to 16 people in the place and once they split up the rental fee, it was pretty cheap. So I told her I was very familiar with the city and asked where the place was located. She told me the street name. She also told me the name of the nice but clueless lady they rented from (my mom's first name is not very common). She said they were planning to add even more people next time they rented. I said, "Oh! You've been renting my parents' place!" Her face went kind of white. She didn't say much else. So I asked my mom if she had been renting to the woman and my mom said, "Yes, they go every summer." I told her about the crowds of people she's been stuffing in there. Needless to say, that lady stopped renting from my mom.