Thanks for the comments. I just checked I have 1000 notebooks
I'm sure there are some workarounds but I'm reluctant to do this - I have a good workflow for my new and past projects using notebooks, also for me more fundamental is that I'm paying for a premium service for a solution that is meant to make my life easier managing information.
I'm waiting for a response from Evernote support, I would be interested to hear the justification for this limit