Not a single developer. The game itself is a product of a Factorio factory, the developer simply drags belts around and flips logic switches to produce new features. The automation has reached a point where it has developed Copilot-like AI skills and very little needs to be done by hand now.
Read the development roadmap on the same website before asking such stupid questions. Gaining sentience and wiping out the human race is scheduled for August 2024.
Initially it was one developer but it hasn't been just him for quite a while. Nowadays they are a team of more than a dozen I think.
They have poured a lot of work into making their game run efficiently. No matter how much they do that, players will always expand their factory until the CPU cannot keep up.