Is it known how many secret hats exist in this winter bash? What about last year?
Also, are hats being added during the event? Should the Stack Exchange team come up with new ideas?
Yes, it is known how many secret hats there are this year – pretty much any employee has access to that information.
All hat orders are finalized before Winter Bash begins. Due to the lead time required in hat manufacturing, we're unable to product new hats once the event begins. New ideas might be saved to jumpstart the trends of next season.
Based on core.js, it seems that there are 30 standard hats and 8 secret ones. This is assuming hichris123's deductions were correct.