If you have your own tried-and-true recipe, how many people does it feed? Divide 25 by how many folks your recipe will feed. Now, take this number and multiply all the ingredients by that.
Of course, you're going to need a HUGE pot to cook that much soup in. In fact, your home stove may not even be big enough to handle that much soup. You may have to make it in batches, and store it.
I've worked in restaurants that make soups in 4-gallon batches. We used a trunion (tilt) skillet to make it. That piece of equipment is big enough to fit my 20-month twins sons, 6-week daughter, and maybe my 6-y.o. stepson into.
Peace, Love, and Vegetable Rights!
Eat Meat and Save the Plants!