Hello Yoloson,
Thank you for using Apple Support Communities! I understand that not all of the photos on your iPhone 6s Plus appear on your new iPhone XR after it was restored from an iCloud backup. I would be happy to help provide steps to take next.
Do you have iCloud Photos enabled on the iPhone 6s Plus? Here's how you can check to see if that's turned on:
Set up and use iCloud Photos - Apple Support
If iCloud Photos is enabled, the photos are uploaded to iCloud and will not be contained in your backup. I suggest checking that the new iCloud storage plan is reflected on the iPhone 6s Plus, the iPhone is connected to Wi-Fi (if it does not have an active cellular data plan), and that the battery is charged. This will help ensure that your photos are uploading to iCloud from that iPhone. You can also go to Settings > Photos, then check if it shows the upload status. Once they have uploaded to iCloud, the photos and videos from that iPhone will automatically sync to the iPhone XR provided the same conditions are met.
If you don't use iCloud Photos, then the photos will be contained in your iCloud backup. You may need to create a new backup on the iPhone 6s Plus, then restore the iPhone XR from the new backup to see if all of the photos appear. This would erase any new data saved to the iPhone XR:
Keep us posted on the results.
Cheers!