The order number on the Apple App Store can be checked in the payment receipt sent to your Apple account's email or directly on iTunes.

[How to check App Store payment history on iOS/iPadOS mobile devices - Method 1]
Open the App Store app.
Tap on the "Today" profile icon.
Tap your own Apple ID and enter your password.
Go to "Purchase History".
Tap on "Change Filters", adjust the filter settings to "Last 90 Days" for date range, "Paid" for cost, and "In-App Purchases" for type.
Touch "Done" to apply the filter settings, then tap on "Total" to view the confirmed order details.

[How to check App Store payment history on iOS/iPadOS mobile devices - Method 2]
Go to Settings.
Tap on your Apple ID at the top, then select "Media & Purchases".
Select "View Account".
Go to "Purchase History".
Tap on "Last 90 Days" for date range, "Paid" for cost, and "In-App Purchases" for type.
Touch "Done" to apply the filter settings, then tap on "Total" to view the confirmed order details.