The suggestions Alex provided you are sound much more like suggestions for sizing an enterprise environment rather than a hotel environment.
Although I dont claim to work in that industry, I believe the requirements are different.i.e.
Is the hotel targeted towards business customers or vacationers?
I guarantee that vacationers will use the phone a lot less than business folks so if you are working on a phone system for the “fun and sun” in Florida the type of traffic they will see will be different from the Hyatt downtown Chicago.
Also 99.99% availability has more to do with the system and not the trunks. That type of availability means the system should be down no longer than 53 minutes per year.
IME rules of thumb are quick and easy but its better to get the right information first and the customer will be happier in the long run. You will be able to back all your decisions with data.