How much space do I need on my storage to be able to upgrade to Catalina OS

From: How to upgrade to macOS Catalina - Apple Support

"Your Mac also needs at least 4GB of memory and 12.5GB of available storage space, or up to 18.5GB of storage space when upgrading from OS X Yosemite or earlier."

I wouldn't try it with less than 30GB, temp files & such will use much more space than the install ends up using.

You'll need about 6.5 GB of space just to download the macOS Catalina installer. After that, the amount of space needed is dependent on the type of installation you're performing: a clean install or an upgrade install.

smart

You're doing great, glad to see you helping out here... keep it up. :)