As a default, GarageBand records on the selected Track. If you want to record multi-track (multiple inputs on different Tracks at the same time), then you have to make the "Record Enable" Button visible on the Track Header first.
Control+click on the Track Header and choose from the Track Header Components the Show/Hide Recod Enable. Tis is also available in the Main Menu Track ➤ Track Header Components ➤
Once visible, you can select that Button on each Track that you want to record on. Make sure that each Record-enabled Track is set to a different Input Channel.

Hope that helps
Edgar Rothermich - LogicProGEM.com
(Author of the "Graphically Enhanced Manuals")
http://DingDingMusic.com/Manuals/
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