Can you Markup more than one photo at a time?

Not with Photos, no. You can only open one photo at a time in Edit mode, and you need to be in Edit mode to use the Mark-up extension. There is no batch mode for photo editing extensions.

You can either use the Mark-Up extension, or you can use the command "Image > Edit With" and select the Preview.app and use the Markup tools in Preview. But either way you cannot batch process the photos in Photos.