how much disk space is needed to download Xcode?

Do the math.

10.46gb spare is barely enough just for the OS to create and utilize swap space, much less download (4.5gb), unpack (5.5gb) and install Xcode (9.72 on my drive) all at the same time. Approx. 20gb for that process, plus10gb that the OS is using = 30gb.

See this recent thread:

https://forums.developer.apple.com/thread/18387

So get busy clearing off space, be sure to empty the trash, and start looking for a larger SSD...calculate how much capacity you need, then double it. Don't forget to add a tested and reliable backup while you're at it.

I have 41.82 GBs free but still unable to install the Xcode 12.5