A lot as in, well depends. It's hard to explain-
See You have a 30 gallon tank. Each cory needs what 25 gallons? So if you get 5 their gallon usage would be 150 gallons plus the need for the other fish. That would mean you would have to use a 350 GPH filter. And if your substrate is going to be more than 1 - 1.5 inches thick you will have problem with detritus. unless you accompony your main filter with an undergravel filter being it be gravel and not sand you will keep things clean. Than theres the issue of live plants, and cories are bottem dweller from what I have seen. So those tablets you throw into the ank for the bottem guys get hidden away sometimes which would cause clarity problems. So why not the school of tetras?