As tiktok said, their over all consumption will remain roughly the same in a 24 hour period. Some days perhaps a bit more, some days a bit less.
What you will likely see is that they will take more at a feed as they start to decrease the number of feeds in a 24 hour period. For example, now they may have 3 ounces at a feed, but in several weeks time they may start to want 4 or 5 ounces, but they will be having fewer feeds in a day with the total for the day remaining about the same.
Are you supplementing to be assured of how much they are taking in or is there a specific reason?
Frequent feeds will be key to production and maintaining supply. As they go through growth spurts that will help, but you can also insure your supply by letting them have their fill at the breast before offering any supplement...or better still, if you feel they need a bit more, just move them to the other breast or back to the first. Pump after a few of the feeds on a regular basis....some women find they will start to produce more at those particular feeds because their body has been programmed through demand. Mornings can sometimes bring an increased amount from pumping as prolactin levels tend to be higher at night. You may want to store some for later when they may want more at a feeding.
In most cases, with continued babies at the breast, you will not have an issue with maintaining supply while also expressing sometimes. If you are expressing only, you may indeed run into some supply issues at some point as generally your babies will move the milk more efficiently than the pump will. Consider the option of doing some of both. You will be better able to maintain your supply, and you can still have the option of letting others help you out now and then.