Will upgrading my Mac result in the loss of my data?

Normally, nothing untoward will happen.

However, nothing is 100% certain, ever.

Backups are a necessary part of life. You really, really should always have at least one, preferably two; one being off-site.

There's one scenario in which the answer is a definitive "yes" : obsolete applications.

As Apple is removing support for 32-bit entirely, you may find you have some specialized application which is no longer maintained, meaning there is no 64-bit binary available. If in addition you have data files in a format proprietary to that application, then once the application is unable to run, you have no way to access the data files.

I could say some nasty things about obsoleted proprietary data formats (Microsoft Office, I'm staring straight at YOU), but you get the idea here.