It depends. April has about half the arrivals in Santiago as May, but the May arrivals probably started the pilgrimage in April, so you need to include the time lag in your considerations. May is about half as busy as the peak months of July and August. Traffic is up enormously over two years ago, so anecdotal evidence from 2014 and earlier is not what you will encounter in 2017. This year will be up substantially from 2015, and there is no reason to think that next year will be any less busy. Crowds are beginning earlier and lasting longer as pilgrims try to avoid the busiest months. September was once ideal; it has been pretty much ruined by late-traveling pilgrims.
If you stop in the early afternoon, it is unlikely that you will have a problem finding a bed. If you plan to rise late, walk slowly all day, and arrive late, you should expect to encounter problems some places. The choice really is between wandering a town with your backpack looking for a bed, or using a phone to wander the town either that day or the day before. I can tell you from experience that the stress level is higher when you are hot, tired, dirty, and hungry and find a place full, then face walking around town or to the next village, than if you call ahead and reserve a bed. Almost all places will insist that you arrive before 4 p.m. to claim your bed. They may settle for a phone call on the day of the reservation that you will arrive after 4 p.m., and hold your bed longer.
If you burden yourself with a lot of expectations that you will be destroying the ambiance of your pilgrimage by planning ahead, then you may be setting yourself up to be miserable. You can choose to be miserable (many pilgrims do), or you can choose to exercise your free will and do the little things that help you adapt to an experience that will not adapt to you! I guarantee you that the pilgrimage, the country, and the other pilgrims will not be adapting to your will or expectations. I have found that I am much happier when I accept that fact. An irritable
hospitalero is going to be irritable regardless of whether you like it. An
albergue is going to be full regardless of whether you wanted to stay there. Pilgrims are going to snore regardless of whether it keeps you awake. You cannot control anything except yourself, and most of what you can control is your attitude and expectations (even your blisters may defy your plans, preparations, and treatments).
If you do not want to reserve ahead, don't. But then don't make yourself all grumpy because everyone else reserved ahead and you did not find a bed. Accept the consequences of your choice and move happily on.
