How easy will it be to book hostels as I travel in South Eastern Europe in august?

Hi all,
I am going to Eastern Europe in august, and the weeks surrounding it, and am wondering how easy you think it will be to book hostels as I travel. I have travelled before but that was in Latin American and off peak so I could decide where I wanted to go and stay even on the day before. However, in Europe at august I can imagine it’ll be much busier, especially with hostels having shut down after covid. I have already booked my first hostel in budapest as it was all booking up quickly, but I’m not sure less visited places such as Skopje or Sarajevo will be as popular.
If anyones been before, will it be possible to decide my next location spontaneously, all will I need to book all my hostels now? Thanks a lot

How easy will it be to book hostels as I travel in South Eastern Europe in august?