Yeast Questions

Okay, so I've worked with yeast starters to make small batches of beer, but now I've realized I don't know a crucial piece of info for if I need to make a larger (MUCH larger) batch. Depending on how long I let a starter sit, how will I know how much yeast I've created? Can I make a massive batch that will stay viable, and if so, is there a way of finding out how much yeast I have per volume? Obviously I know that will depend on how much water/sugar there is being used, but since the batches will be over 100 gallons, it's a bit more of an upscale that I think dumping 60 packets of US-05 would be practicality-wise. Any help would be great!