Definition of Stage
What you will want to do is modify the Opportunity Stage pick list to reflect the way YOU sell your products and services. If you only have five stages today, I suspect someone at some point parred down the pick list that comes "out of the box."
You still need a Stage that counts towards a "Closed Won" (100% Probability) Opportunity and you will need at least one Stage that counts as a "Closed Lost" (0% Probability) Opportunity. Any other Stage values are up to your discretion.
At the end of the day, the Sales Process in Salesforce (represented by the Opportunity Stage pick list) should be support the way you sell your product and services. Don't let Salesforce dictate your sales process. There is no set number of stages. You can have a two step or a ten step sales process if you like!
Generally accepted stages would be as follows
This should cover most stages for pushing through a deal in a CRM. Although depending on the product their may be a longer qualification, or negotiating process for example which you would break down to be more detailed and get better reporting from that data.
For more insight I can be reached via email firstname.lastname@example.org