I'm trying to find rice in bulk, I want to make 5 gal. of Sake at a time. how much rick should I use?

25 lbs of polished rice will make more than 5 gallons of finished sake (assuming additional 6.5 lbs of koji rice culture) if your equipment is good and you have the needed fine grade cotton cloth to hang your solids up to drain well. Everyone on Amazon who is doing the homemade Sake thing is using a standard Calrose grade of medium grain sushi rice. Because that is what is produced here in the US for sushi rice. BUT that is not even close to the special varieties of rice grown specifically for sake in Japan. Those rice varieties are only used for sake making and are expensive in Japan and not sold to rice consumers as people in Japan do not make their own sake at home. Even then, the Sake rice is aged and then specially "rice polished" to remove a significant amount of the outer endosperm when making a high grade of sake. Standard short grain sushi rice is polished so that maybe 10 percent of the outer endosperm (white part of the grain just below the rice hulls and brown layer of brown rice) is removed. BUT sake grade rice is polished to a minimum of 30 percent outer layer removed and sometimes for the highest grades of sake up to 70 percent is polished away. This rice dust is used for other purposes but not for sake. I think this is done to make a cleaner tasting sake that is held in high regard in Japan. Also note that the white or yellow Koji is VERY temperature sensitive so the best Sake makers are all in the northern island of Japan where air temperatures are much cooler. If you make sake at a warm temperature, the result will have strong off flavors produced by the Koji working at a high heat level. Should you wish to do your own rice polishing at home to make your sushi rice more like a higher grade sake rice, you can buy a special japanese rice polishing machine to do just that. You will need a autotransformer to convert Japanese voltage and frequencies to US but the whole rig will work and can be bought from Japan sellers on Amazon or eBay.

Do you simply grind this and place it on whatever you would like to ferment? Does it have the aspergillosis already in it? Please advise.

I use Miyako Koji to make amazake. It's tasty with natural sweetness. The Amazake is very good for you as long as you don't drink too much. High in calories and high carb.