Although it may seem somewhat intimidating, and complex, recording multiple tracks in GarageBand is rather straightforward.
First we'll assume you have an interface, or have created a new aggregate device, that offers you two or more channels of audio input. Audio interfaces may offer dozens of channels of input, which GarageBand can use on any of up to 8 tracks at once. Each GarageBand track can be assigned any single channel, or pair of channels.
Additionally, each channel can only be assigned to a single track at a time. So if you assign channel 1 to track 1, and also assign channel 1 to track 2, you will not be able to record on both tracks 1 and 2 at the same time.
To assign a channel to a track, click in the track header area to select that track
Click the button to open "Track Info", and at the bottom of the window you'll find the controls for selecting the channels you wish to use for input
On this system I have 15 channels from which to select (remember, only 8 tracks at a time)
And for this project I want to record 4 tracks simultaneously. So for my first track
I'm going to select "Channel 1 (Mono)" because I'm recording from 1 mic in each channel
I set Track 2 to Channel 2, etc, all the way through my 4th track, so now my timeline looks like this
Every track on which you wish to record must be "Record Enabled", and we do this by making sure the button is turned on for each track. Which track is selected is not important, just make sure that the record enable buttons on each track are turned on
And you are now ready to record on to 4 separate tracks.
One point worth mentioning, if you click on a track that is not record enabled, it will become record enabled, and all the others will be turned off.