In Switzerland, there is no seat guarantee on trains. Trains are a means of mass transport. However, there is a transport obligation for railway companies. In the unlikely event, you would have to stand until the next stop.
From work experience, I can tell you that there will always be a free seat for a single person. Simply avoid the centre of the train and get on it on either end of the train, in the appropriate class.
The SBB Mobile App helps you find a seat: when you click on the train connection, to see the journey details, there is the option to see the trains’ composition, see screenshot below. As you can see from my example, while the coaches in the center (Sektor B) have a high occupancy, the coaches in the front (Sektor D), or sometimes in the rear of the train, have a lower occupancy. The train composition is available latest 3 hours before departure.
In addition, for IC trains there would be the possibility to make an optional seat reservation, which I, personally, find unnecessary for just one person travelling. See more information here: Seat reservations in trains and dining cars | SBB