If you plan to mess with the building setup, then I suggest the number of rooms be the least of it. In order to have the buildings realistic as far as I see it, there are a few criteria I can think of:
1. Size/Area, this is the square footage of a building/land, thus affecting how many rooms can be built for that building.
2. Quality, The difference between sectioning off a warehouse for your brothel, or buying a mansion.
3. Area of town, If you build a fancy brothel in the slums, you're still going to mainly get scumbags, whereas a shack amidst the town mansions probably won't get much business either.
I would also suggest that there be multiple options at various levels of influence. You may be able to buy the small, somewhat fancy townhouse and turn it into a brothel, or buy the larger but less attractive apartment building and therefore get more rooms at a lower rate of customer attraction/customer pay grade (which would be another idea, richer customers that pay more for better whores, poorer ones that can't afford the good ones-careful where you put them)
You might want to go a bit further and have a "Releator's Office" that rents or sells buildings. Rent should be 1/100th of the cost of the building. Selling a building should give you an option to take half value now or wait 2-12 weeks to get full value. Buildings should have fields for neighborhood (other buildings in area: residential, buisiness, docks, industrial, resort, entertainment, religious), income area (low, middle or high), starting and max rooms for whoring (current/max), appearance rating (affects income level of customers, the better the appearance the better the customers attracted), number of possible amenities and any current amenities (max number: current amenities). Price (rent/own).
Buildings shouldn't just stay available for sale, but should be like the slave market. Everytime you visit new buildings are available. Prices should be determined by the combine value of all other factors. The neighborhood and income area should be the leading two values with rooms having a smaller effect. Appearance and amenities should be adders. So some formula like: [area X income X (current + max / 2 ) ] + appearance + amenities + amenities value = property value.
Amenities include special rooms that can have a positive (or negative) effect on the brothel. Some amenities would earn income, others would come with weekly costs. Some amenities would require some girls to work the amenity, others are benifits from just being there. Some examples would be: adding a bar (requires 2 girls min/6 girls max. generates a seperate customer list and generates a small profit, attracts customers), restraunt (like bar), stripping stage (like bar), garden (adds to building's appearance, raises girl's stats), gambling (requires workers, may loose money but tends to gain a profit, doesn't attract customers but instead gains more money based on the brothel's number of clients), drug den (lowers girl's resistance to performance, lowers girls health over time, makes a profit, lowers attractiveness of the building but raises the number of customers, possability of being busted by police/guards), gym (girls stats slowly improve, girls need more rest), study (girls stats slowly improve, girls need more fun), pool (girls have more fun, small chance of stat improvements), clinic (girls are less likely to be injured, get pregnet, or suffer from other bad health effects. Girls recover faster from injuries. Has a weekly cost for medicine and a nurse/doctor).
Some amenities should affect all of the brothels. For instance if you build a clinic the other brothels could use it too. Generally the effects of each amenity should only affect the girls in that building. Also you should be able to assign more girls to a building that it has rooms, but only allow prostitution equal to the number of rooms availalbe. Maybe max girls per building should be max room+max amenities because girls could use the potential room to live in even if it isn't ready for business use?