Where can I find the “SIM” number?

Okay, I understand you asked for the SIM card in your iPhone. If you want to see the E-SIM number of your Apple Watch, then you should be able to see it in the app. But i can see the field is blank.

Try on your Apple Watch:

1) Press the Digital Crown on your watch to see your apps.

2) Tap the Settings icon.

3) Tap General.

4) Tap About then scroll down. Look for the ICCID field.

Is your Apple Watch with Cellular connection? If not, you will not be able to see any SIM card number.

If I misunderstood you anyway, and it actually IS your iPhone's SIM card number you want to see, then leave the Apple Watch app, and go to your iPhone's settings, and again -> General -> About -> find ICCID.

On your iPhone, go to:

Settings -> General -> About. Look for ICCID - this is your SIM card number.

Most often you can also find the number on the sim card itself.