I would like to reduce the number of lines calculated by the engine to speed up the analysis but I cannot find the appropriate option for that. Does anybody know how to do this in Scid vs PC?
In Scid vs PC, after you turn on an engine, a window with analysis opens. In that window you can see icons like Stop Engine, Add Move, Add Variation etc. (see picture below).
Among them, there is a small box with a number (marked by a red circle in the picture below), namely the number of lines suggested by the engine. Adjust that according to your preference. If I understood your question correctly, that should solve your problem. Just for the sake of completeness, I have the version 4.13 of Scid vs PC.
The keyword you're looking for is variations. To save this for future use, make a new analysis engine.
Under Tools, click either Analysis Engine or Analysis Engine #2:
Fill in the fields and hit Configure UCI engine:
Now you can set the Multiple variations variable to any amount of 'lines' you want:
Don't forget to look at all the other nifty variables you come across :-)
This sets the same number as Zvonimir indicated in their answer, but now it can be saved as a standard setting. If you want to use it again, just select the correct engine.