usually the meals are pre ordered., the easiest way to do this is to include a 'menu' in your invite, with the options on (usually 2 or 3 choices, a meat, a fish and a veggie) then the people will tick what they want and send it with their rsvp, easily done and kills 2 birds with one stone.
then usually hw the venue do it is after you have confirmed your meal choices to the venue and final numbers (1 week prior) they always cook a few extra just in case of any problems so if anyone REALLY wanted to change their options there is leverage to do so.
the venue usually work a system before hand for you to add to the placecards (for example red means meat, yellow fish and green veg, and a similar system for the starters and desserts) so each placecard will have a small coloured dot on the back (done by you or a helper haha delegate where possible), so something like purple, red, orange would mean 'this person would like ceaser salad for starter, beef wellington for main, and orange cheesecake for dessert (totally made up but hey! you get my drift) then the waiters make a note of these before the guests arrive so they know that table 6 wants 4 meat, 3 fish and 2 veggie, then they cook! saves lots of time so all the meals can be brought out at the same time and everyone shud be happy (afterall they picked their choices haha)
hope i helped you make it a bit easier!