Has any laptop ever had more then 1 port that allows a power supply to be plugged into it?

That I believe is why there is only one port. It was simpler to put one port in the laptop and build a breakout box for those that need multiple ports simultaneously. The breakout box still only offers one place to plug the power supply in at, while offering video out and USB A.

The alternative would have been two USB C ports on the laptop, then ensuring a power supply is only ever plugged into one, not both. It's doable for sure, but not a path Apple would likely take. (Similar to how dockable laptops do technically have multiple power ports when docked, but the one on the laptop is physically blocked by the dock to ensure someone doesn't plug in two power supplies).

It will be interesting to see how this product evolves. Will USB C take off in general across the industry? Will we start seeing USB power supplies as a way to finally standardize the way we power our laptops, no matter who the vendor is?