We have recieved the following message :

Hi there,

You have reached 90% of the maximum Customer Portal User permitted for this account. View here

Upon reaching this limit, the associated actions will not be permitted in your apps.

We recommend that you analyze this usage and upgrade to a setup that fits your requirements better.

When viewing the link marked "here" there is no reference to the number of Customer Portal Users.

What is Customer Portal Users actually refering to and where can I see the pricing structure and tiers ?