Bulk renaming files can be a time consuming pain without the right tool. If it's a problem you face often, you've probably already got a favorite batch-renaming app residing on your Mac, but if not, Quick File Renamer might be a good place to start. It has a user-friendly interface with straightforward options -- "idiot proof" you might call it.
Add the files that you want to rename into the "Before Rename" box by using the file finder built into the program or dragging and dropping from Finder. You then select the kind of renaming action you want from the presets, input your custom name, set whether you want to save the renamed files in the same location or another, and whether you want to delete the original files.
If you want to do something a little more complicated, the advanced tab lets you change or add numbers to an existing series or clear the name entirely. You can also modify things such as upper or lowercase names, add dates, suffixes or prefixes, remove or replace sections of names or add audio or photo tags. A comprehensive help system will guide you through the more complicated actions, which is accessible via the good old F1 key.
Quick File Renamer comes in two flavors -- a free 'lite' version, which lacks some of the more advanced renaming and tagging options, but will rename and append file numbers without issue; and a fully capable paid version that'll set you back US$1.99. There are quite a few other batch renamers available for the Mac including Renamer, Name Mangler and Photo A.K.A, while you can achieve the same result with a bit of Automator magic. But if you're looking for a decent free batch renaming app for the occasional file renaming task, then Quick File Renamer Lite is a good start.