Considering you are new and you do not want to upset the apple cart so quickly, approach this happening as a teaching moment, and no sooner they leave, it is time to make adjustments. You will loose some battles at first, but what you learn from them is what will count most in the future.
1. You have a somewhat of a remote place; a gift and a curse when it comes to controlling guest numbers. You need a way to tell who is coming and going is in your place. Cameras are helpful, but it is a touchy subject (see other posts on this very subject).
2. The fact your place is 3 bedrooms, naturally should be 6-persons max place; offering up to 8 (even if it could accommodate them) encourages 'creeping increases in numbers'. Six (6) is enough for such a nice place, 8 should be the exception and the guests will ask at times and permissible when it comes to families for example; they are not the type to throw a party, which is what you want to prevent at all cost. The likelihood of one at 6 guests vs. 8 is exponential, as a general rule.
3. Read, read and read the many discussion here that have covered every host situation over the years; you are dealing with human nature, it hasn't change that much in the last thousand years, so the advice of the last 5 years here is still applicable. 😉
4. Good luck.