Shinjuku-Kamikochi bus reservation required?

Hello,
I'm planning to take this Alpico overnight bus from Shinjuku directly to Kamikochi in early July this year.
http://sawayaka.alpico.co.jp/area/kamikouchi/shinjyuku/

Are advance reservations required, or can I just show up on the day of travel and get a seat?
Is this bus likely to fill up that time of the year?

Thanks.

You are recommended to reserve if you can.
Some of highway bus company allows buying tickets on the day of departure, but its not popular.

The date of the operation in July of this bus, it will be announced in late May. Because there is very popular with climbers in high season, they may be sold out as you say. Relief service might be operated maybe.

Thanks for your reply. I'll reserve a seat as soon as it becomes available.