Shirakawago bus

I'm looking at the bus between Takayama and Shirakawago - some are reserved and some can't be reserved. Is it common for the non-reserved bus to be sold out? Am I supposed to go early to line up?
Thanks

According to Nohibus Company, it said that there will be an increase in the number of buses on the day if it's over seated.
It's recommendable to purchase ticket earlier at the ticket counter in designated place. They say you can ride on it so long you have a ticket for the bus.