My systems scale to somewhere around 200,000+ machines at peak and seem to do fine.
How many customers is that serving, 20 per machine? (Or whatever a customer might be in that case)
"customer" is hard to describe since the buyers are very large organizations. Think multinationals, entities which have air forces and space programs, etc.
It's not a question of number of machines, it's a question of how much load any individual program can serve. Requests per second, per process. If that's O(1k) or less, then sure, do whatever you want, it's trivial load.