You're right. Even if BS have many different compounds, only two are allowed for every GP (I'm talking about dry tyres).

So teams can only choose between this two types, and they have a limited number, so they trend to use both types on Friday, to do the setup of the car for the Qualifying session. They have to keep soft tyres for this session, as well as for the race, where they have to meke at least one stint on every type of tyres.