Hi Luc, yes you're right, you CAN'T pre-book for the municipal albergues (and this is usually also the case for the parroquial or donativo albergues) - it's first come, first served. If you click on many of the albergues in the list on Gronze, you'll see, "Admite reserve: no". And many of the municipal albergues have close to 100 beds! So there's really no point worrying.
That said, the Spanish are on holiday from Monday 15 April, and walking part of the camino is very popular during in the run-up to Easter. Most folk do the last 100km, but the rest of the Frances can also be busy. However, as you can't book for the municipal albergues, you stand as much chance of getting a bed as anyone. And if it's super-busy sometimes they make a plan - when I was a hospitalero in Logrono, we opened new rooms and put mattresses on the floor for the 'overflow'. A good tip is to stop in the smaller villages, just before or after the 'traditional' stages. It's amazing how much quieter these places are.