After each subscription renewal your quota is reset. Then, the total number of tested page views remaining from the previous month in your account will be available for you to use next month.
For example, if you have a 5,000 page views plan and only use 3,000 page views, the remaining 2,000 will remain available to you. So, next month you will have a total 7,000 page views (5,000+2,000) in your account.
However, maximum page views in your account at any given point cannot be more than double of your existing plan. For example, if you have a 5,000 page views plan, your account can never have more than 10,000 page views.