Not in our experience.We are travelling down through Portugal at the end of August for about 10 weeks. We always stay on sites. Will we need to book? Probably just ambling down the coast. Never been before and only planning on doing 100 miles per day max.
There is a chain of campsites owned by Orbitur (https://www.orbitur.pt/en/) most of which are on the west coast
Not all are open in the winter. We've stayed at most of them.
I think they went through a rocky patch about 5 years ago - we haven't stayed on them recently. so worth looking at some recent reviews.
Try Vila Nova de Milfontes - 2 campismos there.
P.S. we once spent 14 days moving along the Alrgarve each day and had only gone 60 miles in that time.