How do I label multiple Photos at one time with same label?

You can select several photos in Photos at once, then open the Info with ⌘I or the (i) button on the toolbar.

Now you can type a title into the "Add Title here" field, or a keyword in the "Add Keyword" field, or enter a description. This will change the metadata of all selected photos at once. to the same title, etc

But Photos does not have elaborate batch changing tools like iPhoto has. You can only add the same same title, keyword, description to several photos at once. If you want more control, for example add a running number, you can do that by using an Apple Script.

For example, here is a script by Old Toad to change the title of selected photos to the capture date with a description and padded sequential numbers:

Batch Change Title to Capture Date-Descri… - Apple Community

More scripts are here: Thematic Index to Photos for Mac related … - Apple Community

And compiled versions of the scripts, ready to run as applications: http://www.oldtoad.net/TT/No.P01.html