There’s currently already over 200 registered! If you are still considering whether to come or not, better decide rather sooner than later! The earlier you register, the better the chance to get the materials (which may include e.g. badges and t-shirts).
Some sites are already reaching their limits, while some still have more room. Helsinki site has been popular and just got fully booked, but you may still register and get in the queue. First come, first served!