how much available storage do I need to upgrade from 10.14.6. I have about 16.5 GB available
can I do it?
MacBook Air 13″, macOS 10.14
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MacOS Catalina - Technical Specifications
Per the linked document above:
"If upgrading from OS X El Capitan 10.11.5 or later, macOS Catalina requires 12.5GB of available storage to upgrade. If upgrading from an earlier release, macOS Catalina requires up to 18.5GB of available storage."
I suggest you do your best to create a storage surplus of 40-50GB before you upgrade. If that requires that you purchase and external drive and offload some data – photo libraries, music and videos – then you should do that. MacOS doesn't like to run low on disk space, and with only 16GB currently available I think you are standing on the brink of trouble.