That is one large tank! If discus will be the only large fish in there companioned with a couple of shoals of small fish, at adult size you could easily keep 25 adults in there. To start with 2.5" fish with the hopes of ending up with 25 healthy large adults, my recommendation would be to get at least 30 juvies. You may lose a couple along the way, you may end up with a runt or two that you will not want to keep. You may wind up with a super aggressive discus that you want to rehome. Sometimes fish die unexpectedly, even discus, so I always get a few more fish than the number I ultimately want to end up with. It is far easier to avoid cross-contamination and disease introduction to get the entire group at the same time from the same source. While they are that small, your ratio of fish to total gallons of water will be such that you certainly won't have to be doing water changes every other day.....depending on how and what you feed. Juvies need to be fed well no less than 4 times/day and ideally 6 times a day. At an average temperature of 86F their metabolism is very high. Done correctly, you can expect to see a growth rate of an inch a month for the first couple of months. There are some strains that tend to grow a little slower than others. I have a pair of albinos that are growing very slowly. My most important piece of advice to you before you buy is to buy from a reputable breeder/importer. Do not buy from your local pet store where you are pretty much guaranteed to wind up with very poor quality discus that will never reach full size if they even live long for you at all, discus that are infested with internal parasites which will be expensive to eradicate if even possible. If you seriously plan to purchase them, I would be glad to give you recommendations for realiable sources in your area if you want to pm me. If you have never kept discus before, it is certainly not impossible, but please do lots of research on their needs and their maintenance before you start.