The easiest and most descriptive option is to postfix the date instead of a counter. That way not only do you know "3 came after 2", but you know when each backup was actually saved, and you never have to worry about duplicates.
To do that, just change your tar command to:
tar -cvpzf /site_backups/purpleearth_$(date -u +%Y%m%d_%H%M%S).tar.gz /home/purpleearth/public_html
Of course you can change the format code to whatever you wish, use "man date" for the available options. The "-u" flag uses UTC for the timestamping rather than your current local time.