Altamira Cave - advance ticket?

Perhaps this website will help
https://archaeology-travel.com/news/altamira-cave-re-open-for-visitors/

You buy the tickets to the cave-replica at the onsite museum. They have plenty of tours every day no need to buy in advance. The actual cave is not open to the public so the tour is of a 1-1 replica, which is very detailed and well done.

There are other caves in N,Spain where you can actually go in and see the art in person. Google Spain cave art. We visited Tito Bustillo excellent,!
Brad

Thank you all for your information and advice! Very helpful!
-Bill

To be honest, I find no reason for visiting a museum and a replica that is full of people and it´s not the real thing, when nearby you have caves like Covalanas, Las Monedas, El Castillo, Cullalvera, Hornos de la Peña...that are very real and display wonderful cave paintings (except Cullalvera, but in any case it´s a spectacular cave).

To be honest, I find no reason for visiting a museum...

The Altamira museum itself is actually quite good and does a great job telling the story of the area in prehistoric times using many different real artifacts they discovered. Being literally right next door to Santillana del Mar, I don't see why you wouldn't combine the two.

Thanks to all. We will be in Santillana del Mar for 2 days, and plan to go to Altamira (museum and replica). We'll also try to visit another cave, time permitting, on our drive back to Bilbao.

https://cuevas.culturadecantabria.com/en/, this will be very helpful, as you need to make reservations well in advance (number of visitors is limited).

Thank you, @MikelBasqueGuide!