Surdan, when you have enough supplies to travel and you finish a level, you should see a clipboard or a postcard on your left saying "Travel." Click it and choose the next city.
Well, if it says you got 499, you got 499. No ifs, ands, or buts about it. Try again, or shut up and deal with it. Besides, I doubt you counted every single headshot you made.
Molten, I'm thumbing your comment down because of the fact that you don't know of the existence of the second last stand. Here's a link:
http://www.kongregate.com/games/ConArtists/the-last-stand-2