@Nadaav0 I had a similar experience. I only saw one (out of the 4 reserved guests) at check in and only 2 at check out. I knew there must have been extra people since extra pillows, blankets were pulled out of the closet and the hot water heater was turned up to the hightest possible heat. Since I didn't discover all this until after they left and since I could not prove they had an extra person or two, I just chalked it up to experience.
Now, with each reservation request and inquiry I require the guest to certify the number of people staying. I also state that only registered guests are allowed to stay (4 maximum, no exceptions). Extra people will have to find accommodations elsewhere. At least that puts the guest on notice that you care and will take appropriate action if there are more than what you agreed to.
Since you only suspect there was an extra person (as did I) the only thing you can do is message the guest and ask them to confirm how many actually stayed. If they deny the extra person, well, I don't think you have much of a chance collecting an extra fee.
If you want to discuss this with Airbnb, click on the link below to see all the ways to get in touch with them, including phone numbers.