Everyone has different levels of comfort with hygiene and convenience. One could travel with even less than you propose, but it probably wouldn't be comfortabl

Everyone has different levels of comfort with hygiene and convenience. One could travel with even less than you propose, but it probably wouldn't be comfortable and your mates in a hostel wouldn't appreciate your presence. 35-40L seems reasonable to have enough clothing to do laundry ~1x per week of travel and still carry gear.