4 GB minimum (8 GB highly recommended to prevent latency).
Downloading for Mac OS X El Capitan (10.11.6) can be difficult because the current Mac App Store version requires much newer operating systems (macOS 13.5 or later). However, you can still obtain a compatible version using your purchase history or a few workarounds. Method 1: Use Your Purchase History Garageband Mac Os X El Capitan 10.11.6 Download
Note: These links are not publicized to prevent server overload, but they are legitimate Apple URLs. 4 GB minimum (8 GB highly recommended to prevent latency)
Some users have reported issues with GarageBand loops not installing properly on El Capitan. If you encounter this: Garageband Mac Os X El Capitan 10.11.6 Download