@Valerie-Et-Loic0 When you book an Airbnb, only the guests who are listed on the reservation and paid for are allowed to stay at the accommodation.
If a place says that the maximum guest count is 4, that doesn't mean you can book for 2 and then have 2 more people come. It just means that the maximum number of guests that listing caters to is 4.
And if the maximun guest count is 4, and your booking is for 4, no, your parents can't also spend the night, because that would mean there are 6 people in a house that is only licensed for 4.
Now some places charge a price for the whole place, no matter whether it's 2 guests or 4. And some charge for 2 and then add an extra fee for additional guests. You have to read the listing information or ask the host for clarification before you book to know that. But even if the host charges for the place the same whether 2 guests stay, or 4, you still have to enter the correct number of guests when you book.
It is a security issue for hosts-they need to know who and how many will be staying. They also need to know how many to prepare for so they know how many towels and other things to leave.
As far as having your parents or friends over for a meal or to visit, some listings don't allow visitors at all, some might if you explain the circumstances and ask permission. You have to read the House Rules to find out what the rules are and ask the host if it's unclear.
Renting an Airbnb isn't like renting an apartment or house full time with a lease. It isn't up to you to invite others over, it is up to the host to decide.