I'll stay 10 days in Montenegro in August and I was wondering if I should book accommodations in advance? I'll be around Kotor, Žabljak and Ulcinj. Is there a risk to find all places full? I'm not picky (but I don't have camping stuff) and I'll have a car.
Kotor will be pretty full in August. Not saying you won't find anything but you might have to pay more for a still free (and possible luxury) room or get whatever is left.
For Žabljak: There are many accommodations and it's generally less crowded there. Also, it's easier to get from accommodation to accommodation when searching for free rooms, as there are usually parking spaces available.
While not having personal experience with Ulcinj I would put it between Kotor and Žabljak in terms of request / demand in August.