Brass instruments

See: Expanding GarageBand for iOS 2.0 content with an in-app purchase

The basic set for GarageBand iOS includes:

You'll get a starter set of three Touch Instruments (Keyboard, Drums, and Smart Guitar), which contains eight preset sounds and Audio Recorder. With a one-time in-app purchase, you can add Guitar Amp, Smart Keyboard, Smart Bass, Smart Strings, Smart Drums, Sampler, and a total of 157 preset sounds. The purchased instruments appear in the Touch Instrument browser after you purchase them.

You'll find horns in GarageBand on a Mac in the Jam Packs, e.g. the jam Pack Symphony Orchestra. The JamPAcks are availble as part of Main Stage.

What about the IOS version?

That is what the beginning of my post was about - the iOS version simply has only " three Touch Instruments (Keyboard, Drums, and Smart Guitar), which contains eight preset sounds and Audio Recorder."

You can buy "Guitar Amp, Smart Keyboard, Smart Bass, Smart Strings, Smart Drums, Sampler, and a total of 157 preset sounds." with the in-app purchase, but no horns.GarageBand iOS is a bit limited compared to the full version on the Mac - instruments simply need too much storage for a hand-held device,