When would they do this? Don't most rush seats go on sale when box offices open? So you want them to delay a 10am box office opening in order to at 10am first count how many people are on line, count how many rush seats they have that day, announce to the rush line how many seats they have, and then they can open the box office and start selling tickets?
Besides, why would you (you = the theatre) 'turn away' people before they even get to the window by telling them there are no rush tickets for them when they're still on the line? You never know if the patron might buy something else - partial view, discount code, box, whatever. maybe even a full price - if rush is sold out.
And then can you even picture the threads on here if the number is, as you say, somewhat off: "the sign on the window/the box office person PROMISED that there were x tickets, but then there were only x-1 tickets, and so I didn't get a rush ticket this morning!!!!!!"
I will say I walk by Beautiful before 10am all the time on my way to work, and I like the way they do their signs, it says "there will be rush" "there will only be standing room" etc, whatever that day's situation is, that even as someone just walking by, who isn't waiting, I know what's going on, before the box office even opens. Their sign never states a number, though. (At least that I've noticed.)