Do I need to back up my my mac before reinstalling mac OS?

Hi

In theory a reinstall does not erase your user data, but with that being said. I would still recommend making copy of it time machine is very easy to use : https://support.apple.com/en-us/HT201250

Best Practice is to ALWAYS have a recent, local, disk-based Backup at hand.

A macOS install will re-write over 350,000 files. it would not be Prudent to proceed without backing up your files before attempting such a large-scale re-write.

Yes. A problem may occur.

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