Were not aware of any 100k systems but there are a couple 65k, and a bunch of 20-30k. All run off medium powered single servers without any trouble.
I could make an educated guess that a current medium to high end server would handle 100k without any trouble. ATutor has been designed from the start to make efficient use of processing power. But, if you wanted to split installations, you could potentially connect them all to a central authentication server, and have them all operate as one system. There are no additional cost associated with licensing ATutor, so you can create as many installation as you need.