iPhoto had a very handy Batch function where one could re-title a batch of photos while appending a sequential number to the end of each title. Unfortunately this feature is missing in Photos. However, a couple of clever Mac users have written several variations of AppleScripts and Automator Applications that restore this capability*.
I have included a link later in this post to a version you can download and use.
First, though, you need set some things up. A good place to put it is in your Scripts Menu in the top menu bar.
If you already have the Scripts Menu there, fine. If it is not there, go to Applications/Utilities and double-click on Script Editor. in Preferences/General, click on Show Script menu in Menu Bar.
Now that you have the Scripts Menu in the top Menu Bar, click on it and choose Open Scripts Folder/Open User Scripts Folder.
If you wish, you can create a separate folder for Photos Scripts.
Now download the app. After downloading, double-click the Zip file to reveal the app Batch Change Photo Titles. Drag the app (not the Zip File) to your User Scripts. Now you can select multiple photos, go to Batch Change Photo Titles and enter the new Title information.
It may take several seconds for the operation to finish.
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*They had several versions, changing the title to the filename, etc., plus a version that worked with iCloud Photos Libraries. If you want to explore these, see https://discussions.apple.com/docs/DOC-8411 and https://discussions.apple.com/docs/DOC-8414