One parsing rule is responsible for extracting one data point from your email (Customer name, an email address, a tracking number, ...). This means that you will typically create as many parsing rules, as fields you want to extract from your emails. There is no limit on how many parsing rules you can create.
For example if your email looks as follows, this would be a minimum of 8 separate parsing rules.
There could be more parsing rules required depending on your needs, for example Primary Contact Name could be 2 fields (for First and Last Name), Address may be another field where you may want separate values for City, State, Zip etc.