Still one per server even as a Container?
It really depends on what you're going to use your instance for. If it's for development process, then you're free to use as many instances and as powerful edition of Oracle Database as you want (yes, even Enterprise Edition with all features). But if you're going to use it commercially in production, then you need a proper licence. I doubt that XE limited BY LICENCE to one instance on a server, I bet it is a technical limitation