In OS X releases before Yosemite, renaming a group of files at once either meant third-party software or rolling your own rename script using something like Automator or AppleScript.
These days, however, you can just select a group of files then select Rename either from the right-click contextual menu or from the drop-down button marked with a cog icon in Finder windows.
When you do, you get the option of adding text, replacing text, or applying a format such as a name and an automatically incrementing counter.
When you do, you get the option of adding text, replacing text, or applying a format such as a name and an automatically incrementing counter.