I'm afraid in my view it will not be the number of rooms, but the number of rooms occupied with paying guests at a decent rate that will pay the bills so let's go at this a slightly different way.
Will you be mom & pop doing it all yourselves or host & hostess hiring folks to do the work, don't answer to me, but just think about those thoughts. We have 8 rooms, 16 beds, don't serve food, at age 50 wife ran the place and I was extra helper evenings and weekends when not working at day job, worked well for us, at age 75 it seems a lot larger than it once did, but next generation of innkeepers now lives next door, that helps. For me I prefer to do the stress free physical work as opposed to the stress of being manager and bookkeeper.
Next part of that, are your rooms going to be busy everyday, all seasons or busy enough certain times to carry the rest of the year, again, don't answer me, but consider you and the place you choose and the lifestyle you desire. We probably have good and excellent weekend business 8 months out of the year, many weekdays and the other 4 months not so exciting, but the flip side of that is that we are happy, bills got paid and we got to breathe and enjoy life, retiring from one career I assume you desire a little fun time too.
Two ways to look at purchasing, back when I would have said it doesn't matter how much it costs, it's about how much you have left after you make the payments, my wife had the view to choose small, cheap, pay off the loan, after all we got to live somewhere anyway, guess we followed her view, but it has worked well for us.
Hope some of that proves useful to you, I'm sure much will depend on the area of the country you choose and also consider the type of guest you desire as your guests will also contribute or take away from your happy life. My ideal or target guests are just nice everyday folks, middle of the road, I'm not comfortable with the rough and rowdy crowd and don't figure I'm a classy enough guy to be comfortable with the upper crust so consider a place that fits you and the guests that will contribute to a happy life..