As far as I know the criteria can be found at their knowledge base. You have to have certain hardware to be a node. You can look it up if interested, it's rather simple to find. If I remember correctly, 150-200 was their initial plan. Maybe they will stick up to it, maybe not, and will actually increase the number of nodes.
I know the requirements for the owners of the nodes very well. I wondered why this question is still not being resolved. Apparently the software for the nodes is not ready yet.
We have a couple of test nodes currently. They are under testing now. Yep, you are right - time is required to implement the whole system