The Merchant center ID can be found in the top navigation in your Merchant center account.
Step 1. Login to your Merchant center account https://www.google.com/retail/solutions/merchant-center/
Step 2. If you have multiple Merchant center accounts connected to your Google account, make sure you navigate to the correct account - using the top navigation dropdown.
Step 3. Copy the ID next to the accounts name in the top navigation, seen below.