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The spell Create Food and Water says:

You create 45 pounds of food and 30 gallons of water on the ground or in containers within range, enough to sustain up to fifteen humanoids or five steeds for 24 hours. The food is bland but nourishing, and spoils if uneaten after 24 hours. The water is clean and doesn’t go bad.

But from the PHB (p. 185) or basic rules (p. 69), under "Food and Water":

A character needs one pound of food per day and can make food last longer by subsisting on half rations. Eating half a pound of food in a day counts as half a day without food.

[...]

A character needs one gallon of water per day, or two gallons per day if the weather is hot.

^{Emphasis mine.}

Also, from the DMG (p. 111), also under "Food and Water":

The food and water requirements noted in the Player's Handbook are for characters. Horses and other creatures require different quantities of food and water per day based on their size. Water needs are doubled if the weather is hot.

$$
begin{array}{|c|cc|}
text{Size} & text{Food} & text{Water} \hline
text{Tiny} & frac{1}{4} text{ pound} & frac{1}{4} text{ gallon} \
text{Small} & 1 text{ pound} & 1 text{ gallon} \
text{Medium} & 1 text{ pound} & 1 text{ gallon} \
text{Large} & 4 text{ pounds} & 4 text{ gallons} \
text{Huge} & 16 text{ pounds} & 16 text{ gallons} \
text{Gargantuan} & 64 text{ pounds} & 64 text{ gallons} \
end{array}
$$

So, if it produces 45 pounds of food and 30 gallons of water, Create Food and Water should be able to feed up to 45 PCs / Medium Creatures and give drink to 30 PCs / Medium Creatures.

Even more, if we take into account that a ration of food weighs 2 pounds (PHB, p. 150), it should be able to feed 22.5 PC, not 15. Also, if my English isn't bad, steeds are horses, which are large, so it should be able to feed up to 11.25 and give drink up to 7.5, not 5 steeds.

So, finally: How much PCs can be fed with Create Food and Water? Maybe that bland food has fewer calories than typical food?

Spells do what they say they do and specific beats general

There is a general rule about food. There is a specific rule about rations. And there is a really specific rule about the spell - it says its enough for "fifteen humanoids or five steeds for 24 hours". Maybe magic food isn't as nourishing as real food.

The spell specifically feeds 15 people.

Most likely this is simply an error, but if we try to take it literally, then the food is one-third as nutritious as normal food. My preference would be to assume all the food is individually wrapped, because I like the possible side effects of leaving a giant trail of garbage wherever you go.

Another possibility is that it produces 45 pounds of celery.

The water is harder to deal with, since a gallon of water should be a gallon of water. Water consumption is doubled in hot weather, so perhaps it should be read as supplying 15 people minimum, with water to spare in most cases.

Rules like this I usually tend to lean towards the exact language used in the spell/ability. In this case a few things could be said about how the food weight could be increased without adding any additional nourishment to the PCs. Thicker or heavier food, waste with the food (example would be like bones in the meat) you might add that weight but your players wont be eating it (.... probably).

In any case I would stick to the ruling laid out by the spell.

Rations contain all the food needed for 1 person for one day, but they do not define how much food a person needs. The chart does. Rations may contain extra food, or packaging and preservatives that account for the extra weight. Packing something in salt is a lot less efficient than modern preservatives. The spell feeds 45 people.