If we're talking about commercial operating systems then Windows NT comes to mind. Here's what they say: "In 1988, when the project first began, the team compr

If we're talking about commercial operating systems then Windows NT comes to mind. Here's what they say: "In 1988, when the project first began, the team comprised about 20 engineers. By the time the first version of Windows NT shipped five years later, the team had expanded to about 150 engineers as it battled constraints and tradeoffs." https://news.microsoft.com/features/the-engineers-engineer-computer-industry-luminaries-salute-dave-cutlers-five-decade-long-quest-for-quality/ https://news.microsoft.com/features/the-engineers-engineer-c... But I think comparing these team sizes is not very meaningful without at least a cursory look at what is part of the project and what isn't. Is it just the kernel? Does it include an entire graphics pipeline, a UI toolkit, a network stack, device drivers?