I stayed here last week and my experience was as follows: we arrived at 13:30. By then we were no. 5 and 6 in queue. Slowly more people arrived and we waited for it to open at 16:00. In the end, not all beds were filled and we could have been more chill. I also saw spare mats and mattresses.
I heard later from another pilgrim that these spare mattresses were indeed used on another day, and after housing 6 or so extra people someone was sent away, but they arrived late (like 18:00 or so, 2 hours after it opened).
My recommendation based on my experience and what I heard from others, is to trust that you will find your bed there. Plan to arrive between 15:00 and 16:00. A lot of people go straight to San Sebastián and potentially take a rest day there. Others don’t like staying in (non-bookable) albergues, or stay only in hotels. Please don’t rush your first day, it is wonderful! Especially the climb to mount Jaizkibel. Just take your time and enjoy your first day!
Oh and context: there were around 40-50 people in the municipal albergue in Irún my first day.