In NuPIC, we usually define a region as having 2048 columns, which is very small. I don’t know how many different “fields” a real portion of cortex could the same size could process, because we’ve already exited the biological nomenclature by talking about “fields” instead of direct input from sensory neurons. But like Subutai said, anything more than 5 starts getting very slow to process.
I am not sure that hierarchy will help solve this problem, because the encoder input still needs to be fed into the lowest level of the hierarchy together so spatial pooling can apply to all the input together.
If there is input to the system that you know is not correlated with other system input, and each different input is processed by a different region, then yes hierarchy will help with with scalability, but remember we are still focusing on one region.