@Sunil35 You need to set up a separate listing for each of your cottages. You can't list them all in one listing- that will never work. In each listing, only show that particular cottage, unless they all look exactly the same. Then mention in each listing that you have 15 other cottages, in case larger groups want to book more than one of them. You could number or name each separate cottage.
Then, if you want to charge by the number of guests in each cottage, follow @Nutth0's instructions. Or you could choose to just charge a set amount for each cottage, regardless of the number of guests in that cottage.