If you generally have the Facial Recognition feature turned on in your Canto account and when you are performing an upload, you can temporarily turn off the feature in the Metadata Editor upload popup.
Turning off this feature during an upload can help improve upload and processing speed.
Upon upload, scroll to the Facial Recognition for Images section of the Metadata Editor upload popup, uncheck the feature, and you're all set!
If you are performing a batch upload of images of a person or a few people that already exist in your library, disabling the option "Identify and tag new people in Images" would not prevent the system from recognizing their faces and correctly matching them to the ones that are already indexed in your Canto account.
But if you would like to identify new faces along with the ones that already exist in your library, simply select the option "Identify and tag new people in Images" and Canto will identify and index all faces present in the batch of images you are uploading.