how do I find out what my iCloud security code is?

I have never heard of an iCloud security code. Apple has various passcodes and passwords so perhaps you could tell us in exactly what context your are being asked for a code? Maybe you mean this one? -->

Get a verification code and sign in with two-factor authentication - https://support.apple.com/HT204974

"Whenever you sign in with your Apple ID on a new device or browser, you'll confirm your identity with your password plus a six-digit verification code. There are a few ways you can get a verification code. You can use the code displayed on your trusted device, get a text or phone call, or generate a code from your trusted device. If you use iOS 11.3 or later on your iPhone, you might not need to enter a verification code. In some cases, your trusted phone number can be automatically verified in the background on your iPhone." See additional tips at: https://discussions.apple.com/thread/8628973