Hola
In Spain most, if not all, Tourist Informations in towns sell Credentials /'Pilgrim Passport'. I am not sure it is the same in France.
I am wondering if that is what you meant when you wrote Sello? Sello means 'stamp' in Spanish and many (many) places along the Camino it is possible to get stamps in your Credential.
All Albergues, most churches, bars, restaurants, even private people would like to stamp your Credential.
Remember that you want at least one stamp a day to document your journey.
The Tourist Information in Hedaye is :
Hendaye Tourisme
67bis Boulevard de la Mer
64700 Hendaye
France
Monday9:00 am – 7:00 pm Tuesday9:00 am – 7:00 pm Wednesday9:00 am – 7:00 pm Thursday9:00 am – 7:00 pm Friday9:00 am – 7:00 pm Saturday9:00 am – 7:00 pm Sunday10:00 am – 12:30 pm3:30 – 6:00 pm
hendaye-tourisme.fr
+33 5 59 20 00 34
The distance from Hedaye to San Sebastian is 25-28 km, so for a first day, I would not plan on going further.
Your first week, until you get to Bilbao, will be the most challenging week of your entire Camino.
To avoid injuries, you should start slow, adjusting your body to what you have in mind.., and not press yourself to much.
Also, the first week is through beautiful forest and woods, many ups and downs where many pilgrims have had second thoughts about the physical condition needed to complete the Norte.
Luckily it becomes different, and easier, after the first week.
About crowds in San Sebastian and the need to book, you may consult San Sebastian online Tourist Information or call them to ask.
The Film Festival is from 19-27 September, so it is fair to anticipate that most beds are taken, but maybe you can get lucky if you take action now
Personally I would expect all cheap/reasonable lodging for taken as well as the albergue may be overrun by Festival people.
Maybe some other forum member have experienced this a previous year and can advice you?
http://www.sansebastianturismo.com/en/
Check your options to alternative lodging, preferable before San Sebastian and let us know how it goes.
Also, I would recommend you to read up about camino Primitivo, as you will have that option later on your way.
Either follow the coast or go inland through mountains.
You don't have to decide now, as your body and experience will tell you what you want, once you are at the fork.
Buen Camino
Lettinggo