Once I flag a question, does that mean I can never ever ever ever get that flag back? For example I currently have 10 flags on superuser, if I were to flag a post would I permanently have 9? If not, how would I get that flag back?
Flags are reset at the end of each UTC day. You will have new flags after midnight.
You get additional flags per day for every 2000 points of reputation you get and for each 10 helpful flags you have (whilst unhelpful flags reduce your flag availability). Source