Straw man - generally the idea of YIMBYism is to be more inclusionary, so that 3+ stories is allowed not required.
In contrast, SFH-only zoning is exclusionary.
In contrast, SFH-only zoning is exclusionary.
But my point is saying "X amount of homes would be illegal to build" is completely pointless and just meant to rile people up. OF COURSE most buildings are illegal to build today. We don't build like we use to for a variety of reasons. Even if you forget about building code, there's also rules around minimum amount of affordable units that older buildings didn't need to follow. Zoning is a specific one and yes, it's often problematic and regressive. Let's focus on that.
Where are we taking this conversation now? Yes, it would be impossible to use the same physical piece of lumber in two houses at the same time.