This is the first post I've been able to answer from experience. I've had this problem before. For some reason, Access doesn't always save the text you put in the control source, even if you actually save it (if you get what I mean!!) Sometimes you have to re-paste it three or four times. Just make sure you're in Design view and try it again, saving it each time. Eventually, it should work. Also, you may want to try (assuming there's a function for this - check the expression builder) =Count instead, if all you want to do is count the number of reservations. The rest of your code, despite my inexperience, sounds like it should work.
If this doesn't work, post again and I'll see if I can help any more.
I can't actually say for sure that your code is OK, because I don't have a lot of experience with that side of things; but all I can say for now is try out that saving thing, because I'm having that problem myself with a similar thing. Good luck.