The first thing is to determine the quantity of food that you will require and out of that how many meals you want freeze dried. You can carry a combination of frozen and fresh food according to your liking. Most people will want to have meals for breakfasts, lunches and dinners for their trip.
You now have to figure out the calorie requirements according to your route. For instance, hiking in the mountains will require more fuel for your body as hiking and carrying a backpack takes a toll.
Since freeze-dried foods need to be rehydrated with water, it is important to carry enough with you if your route does not have any water sources. However, it is recommended that you choose a route which has access to water.
You will now need to do some research regarding frozen meals if this is your first time backpacking with these foods. Visit a store that has freeze-dried food displays as this will give you an idea of what is available.
It is important that you choose meals according to your liking as after a long hike you certainly will not want to eat something undesirable. Packing your favourite meals should be your top priority.
When buying, note the labels of the packages and take care of the serving size as manufacturers usually over-estimate the number of people who can consume one pack.