Sensors are how a controller board sees the world. Before buying a kit, it helps to know the main families: digital sensors give on/off signals, analog sensors give a range of values, and specialty sensors cover temperature, motion on three axes, gas, and more.
Pick by the project, not by the count
- Line following or obstacle detection: digital and distance sensors
- Weather or environment projects: temperature and humidity sensors
- Motion and gestures: 3-axis acceleration sensors
- Air quality experiments: gas sensors
A good first kit covers one digital sensor, one analog sensor, and one sensor tied to a project idea you actually want to build. That combination teaches the two signal types and ends with something that works.