How much space do I need to make a bootable Catalina volume on my Mojave Mac?
The installer takes 8.1 GB of space in your Mojave /Applications folder when it downloads the installer to your Mac. 

In the case where you install to a brand new / empty volume, you will end up with two APFS container volumes - one for the read only system files and another for the user data.

APFS Volume Catalina - Data         3.7 GB     disk1s5
APFS Volume Catalina                10.8 GB    disk1s6


Doing the math, that's 14 GB of space for a totally empty installation, no third party apps, no user data migrated, no iCloud and just an admin account home folder.

Add an additional 8.1 GB of space for the installer and you're able to start testing with as little as 25 GB of free space on Mojave.

If you're interested in how this split works in practice - I recommend this article highly.


https://eclecticlight.co/2019/10/08/macos-catalina-boot-volume-layout/