What do I do if I forgot to take my SIM card out of my iPhone before sending it out for repair

You do NOT need to cancel the repair and you WON'T lose your phone number. That's your number and no one else can use it, even if you forgot to remove the sim card from your phone.

If they replace your phone, when you get it back, you just get a new sim card from your cellular provider and they'll put your number on the new card. Stop worrying about this.

brookes258 wrote:

I will cancel my repair, as I want to keep my phone number.

What kind of repair? Quite a few Apple repair services are just replacing the entire device. I’d think they would check the SIM tray although I don’t know if they would return it. If it’s something like a screen assembly replacement, it’s most likely that they’ll return it with any SIM card.

I don’t know what would happen if you don’t get your SIM card back, but you won’t lose your phone number if you don’t have it. I’ve known people who had lost or damaged their SIM cards and it’s easy enough to buy a new one from the carrier or from a representative of the carrier. They’ll arrange tying your phone number to the new SIM card.

You don't need to do anything. The only data contained on the sim card is your phone number. When Apple's trade-in partner receives the phone, they'll dispose of the sim card.