Where are the photos missing? On your iPhone, on your Mac, or in both places?
Are you using iCloud Photos to sync your Mac and your iPhone? In that case you must not delete any photos from your iPhone that you want to keep on your Mac. iCloud Photos is syncing your devices and keeping the Photos Libraries on the synced devices identical. When you delete a photo from any device, it will be deleted from the other devices as well.
Deleted photos are saved for third days in the recently Deleted album. Have you checked this album? On your new Mac it will be shown in the sidebar of Photos, in the Library section of the sidebar, if there are still photos to be recovered.
If the photos are no longer in Recently Deleted and are having still Apple Care for your new Mac, contact Apple Support for help asap. Perhaps Apple Support can help you to recover your photos, if something went wrong with setting up your new Mac from iCloud.