How can you find out if my Mac is 32 bit or 64 bit?

The Mac hardware has been 64-bit for well over a decade. The macOS High Sierra is a 64-bit operating system that can run 32-bit and 64-bit compiled applications, provided they are otherwise compatible with it. With macOS Catalina (10.15.7) and later, one encounters support for strictly 64-bit compiled applications and 32-bit apps are disabled.

The responsibility for third-party applications becoming 64-bit rests with that vendor.

Any Mac running Mac OS X 10.13 is already 64 bit.

The problem is not so much 64 bit for the operating system, which your question is about, but whether or not your applications are 64 bit. That's documented more in detail on my user tip:

https://discussions.apple.com/docs/DOC-250001761

Where, and if you can upgrade to Catalina, you may be able to also upgrade to Big Sur or Monterey. A lot depends on the age of your Mac. But the biggest factor upgrading to Catalina or later is if your applications and drivers are 64 bit. Not if your Mac is 64 bit. 2008 and later Macs are all 64 bit. Only the old CoreSolo, or CoreDuo without the 2 in the middle are 32 bit. Core i3, i5, and i7 are all 64 bit Macs.

You are welcome.