Reserved seat for bus to Kamikochi?

I'm going to Kamikochi on 7th June. I will travel via Matsumoto Dentetsu Kamikochi train to Shinshimashima and then take the bus to Kamikochi. I'm not sure about there are enough bus seats for me because that's the weekend.

http://www4.ocn.ne.jp/~majp/nature/kamikochimuseum/kamikochiguide/fork...
what the website explains is roughly:
buy a round ticket from JR Matsumoto to Kamikochi, then return to Matsumoto at JR Matsumoto station. (effective for 6 days)
Dentetsu train depart from No. 7 platform (No.6 will go to the wrong direction). about 30 min.
get off from the first door in the first car at Shinshimashima.
harry to the bus stop. the station staff will check your bus ticket.
all passengers have to get seat, because the bus goes to the mountain area. if full, they will run additional buses, (until all passengers can go ?). if possible, get the seat at the LEFT side. good viewing. it takes about 65 min. may take more, if the traffic is busy.
when you arrive, you have to get a reservation of return bus seat. (only on the same day or next day. ) it is better to return from Kamikochi bus terminal. the reservation is effective, if you take it at the bus terminal. take a seat at LEFT side. (Why ? )