OpenERP is free software. Free as in freedom & free as in gratis
No matter how many users or databases you want to deploy are or what you want to manage with OpenERP you will not (and will never) have to pay license fees to anybody for the sheer usage of the software including all publicly available Modules.
If you need any kind of professional support (what most people & organization will need) or hosting services at any point you can find whomever is capable of providing such services and pay them for these services they provide to you.
Such a service provider could be:
- OpenERP SA
- any of their partners
- any other company or person of your choice
So, the use of the software is free of charge at any time. Whatever you pay it will be a service provided for this Software. And if you need a service or not is up to your own choice.