You won't know for sure until it happens, unfortunately. It's probably worth pumping a bit more to start with. However, most likely the total amount won't change very much. The baby is still eating the same amount of milk, right? The presence of cereal doesn't change that (until it becomes enough to offset the milk needs, anyway).
Normally, babies on cereal start with a very liquid cereal mixture - 1/4 or so the amount of cereal per milk as normal. For the first bit, they may not really eat all that much or may spit it up, so it's not uncommon that some wastage occurs; however, the daycare should know that and not make too much to start with, only an ounce or two at a time at most (probably less).
Once it's been a few weeks and the baby tolerates the cereal well, typically the cereal will help her feel full for a longer time, and thus the baby will not need to eat quite so much milk separate from the cereal; my kids needed less milk overall at this point, but we also added vegetables nearly right away (in fact on the second, we started vegetable before cereal, as our pediatrician suggested). You'll also start adding more cereal per milk, so the total amount of milk used in cereal may not add too much.
Overall, though, it's hard to say, because other factors will matter more - your baby's growth cycle and what happens to her tummy/intestines as a result of eating solids for the first time. She may be more hungry, or less. That will likely matter more than the amount added to the cereal.