I asked "Herself" who does this room night management thigamajig as a profession. She tells me what she does, but I still can't explain it.
The answer you'll get here is simply not a specific number. The answer is: at least the minimum amount of room nights and ancillary revenue to keep the operation functioning. In other words... if you're still open, you have enough rooms booked.
Obviously, there is an economy of scale. Where there are fixed costs, you are better off spreading those fixed costs over the revenue generated by 10 guests versus just six. This scale is very fluid and always adjusting, especially in the micro environment, where it must be monitored and analyzed often and well. In Utila, you have the added luxury of extremely low labor costs. This advantage quickly balances out over the particular disadvantages that you face.
She did a cost/revenue analysis study for a resort in the BVI and easily showed mathematicaly how their current 85 person maximum, if expanded to 115 "pillows", would make them hugely and disproportionately more profitable. All due to pre-existing fixed costs being spread over more guests.
Targets must be set. How occupied would you like to be? Many small resorts that are just starting out get into a bad reputation by attempting to do remodeling or repairs while guests are in the room (or next door, etc.) Having an occupancy rate goal of 100% is a nifty idea, but there must be a planned rotational maintenance schedule that will allow for rooms to remain un-sold while the workment are very busy inside!
Hotel rooms are just like airline seats or cotton candy. They're only good for that set period of time, then they melt away... they rot and become unusable. Either they're sold, or they're lost. Look at the naievete of the thread elsewhere that complained about a liveaboard operator refusing to refund with a two week notice of cancellation. In a large opertaion that lost revenue could be eaten by the operator for "good will". A small resort or ship does not have that luxury.
Once you have decided your minimum target occupancy relative to your need for income, putting "butts in seats" is about the simplest part of the task. The best way to fill a small opertaion is by word of mouth, and these days, the internet can be very helpful, most notably with reports on SCUBA Board. It can also hurt, either with the one (inevitable) disgruntled guest with an email id, or misinterpretation of what people write. You have to understand how to manage your image on the internet.
Neither of us were sure what the original information you requested would do for you, but suffice to say if the opeartion isn't at minimum in the 75-90% range it is suffering from a host of problems. It could be caused by a bad operation's reputation, low consumer awareness (bad marketing, lack of p.r. or being on the leading and unknown frontier of adventure travel), travel/access difficulties, poor stateside reservation service, and these days: anything short of a perfect website (with prompt replies to all inquiries).
