Interesting question. I think it's possible to maintain and even improve the average level of intelligence of employees, but past a certain size it's so ridiculously difficult as to be infeasible, practically speaking. Also, I'm not convinced that it's in the company's best interests, but I guess that depends on what the goal of the company is. If, like most companies, it is to make money, then having too many smart people could be a liability; after a certain point you want to hire people for reliability and teamwork rather than individual intelligence. Even if the company has some other goal, I would say it's probably easier to try to keep the company small and automate as much work as possible than it is to keep hiring really smart people.