Whatever method we use will have some internal flag about whether it is possible to build more units at the moment. I guess we could use adding and removing a resource as an externally visible representation. I think having the "build" button for the unit greyed out with help text about why it is greyed out is also a good representation. For example, in vanilla, you may have the technology to build a horse archer but no horse resource connected up. In this case your city has a horse archer build button but it's greyed out, and when you hover the mouse you get help text "Requires horse resource". So I'd want the Guardian build button greyed out with help text "Not enough available ammunition".