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You can start with ROS-Base and add any necessary packages later, as you need them. Which packages depends on the tutorials you want to follow. I don't think the first few ROS Tutorials require additional packages, but I haven't thoroughly read them in a long time. I think you can get through several with just rqt, but each tutorial should state prerequisites. If you end up not having a required package, you can install it easily. For example, the Understanding ROS Nodes tutorial requires you to install a package with

sudo apt-get install ros-melodic-ros-tutorials