Thought experiment: what if the US government limited companies to a maximum number of employees, say 5000?

There would be new inefficiencies, but it might decrease the possibility or impact of monopolies and encourage competition. There would be a lot more specialized contractor companies. How much of, say, Google's search service or Microsoft Windows could be maintained or developed with just 5000 people?

We would end up with massive conglomerates who owned 500+ companies of 4999 people, and if that didn't work you'd just get simple collusion where everyone knows company A and B are owned by the same person so they try to cooperate even without being explicitly ordered to.

It would likely just encourage convoluted corporate structures that would function more-or-less equivalently to the ones we have today.

There would be 5000 car manufacturers

Canada too.