Ad account IDs are useful when an advertiser has to maintain a client’s account or submit a support request to Facebook. This is crucial for advertising agencies, which frequently operate with numerous organizations that require urgent ad management. Finding your ad account numbers only requires two quick and simple actions, adding a partner to your Facebook ad account on Business Manager. Find your business manager id on Facebook https://business.facebook.com/  by following the steps listed below.

Step 1: Click “Ads Manager”

Visit  business.facebook.com
Click the “Business Manager” tab, then the “Ads Manager” tab that appears on the drop-down menu, to access Ads Manager.

Step 2: Click “Account”

Click the “Account” tab to see all of your account IDs. Each account name in the drop-down menu will have your account IDs next to it.

All done! To assist you manage your Ads account, send this number to your advertising agency.

Source: https://www.advertisemint.com/how-to-find-your-facebook-ad-account-id/