The problem isn't really I/O (I've used a 32R with a NSB 8.8 many times), but max numbers of channels.
There is nothing stopping you from running a setup with multiple rackmixers as submixers and e.g. send the master or a bunch of stems from each submixers to the "main mixer" over AVB. However, it sounds like what you are really asking for is an SL64R...