This is always an interesting topic. It's amazing that over the years the rule of thumb has never changed. 10,000 per screen... yup, that's what it has always been.
My situation. Single screen in a town of almost exactly 10,000. BUT, it's a suburb of a metro area. Population within 15 miles is 600,000. The total number of screens serving that population is 81 indoor and 3 drive-in. 3 (all single screens) of the 81 are subrun, the rest are all first run. I'm one of the subruns.
Another interesting question would be: How many people can you expect to draw per 10,000 population? I average between 1200 to 1500 per week, but on blockbuster pictures go as high as 2500 to 3000. Of course my patrons come from all over the metro area, so I have no idea how it applies to the 10,000 population rule. If you are in an area such as leeler is where you are the only theatre in the drawing area, you can provide better information as to the average attendence per 10,000 people.