can I purchase additional software space for my mac?

You can use iCloud to store some files and folders.

What you can do with iCloud storage - Apple Support

You could also purchase an external drive to store apps and folders.

Connect and use other storage devices with Mac - Apple Support

If your Mac is upgradable you can buy a larger internal Hard drive. Install it or have t installed.

The Internal Drive as a " Limited Total Capacity " - the only was to have More Space is either to have the Internal Drive replaced for by a new computer with More Drive Capacity .