You do not need an external USB audio interface. In this case, you can use the existing headphone output.
However, if you need more than two audio channels, you can connect any USB audio interface that is "class complient".
To connect a USB device you also need a USB to Lightning Adapter (eg from the Apple Store). This is also available as a Lightning to USB Camera Adapter, which at the same time supplies the iPad with power.
- RME Fireface UCX
- RME Fireface UFX
- iConnectivity iConnect Audio 4+
- Focusrite Scarlett 18i20
- PreSonus Studio 192
- Motu Microbook IIc
- Behringer UMC404HD
- Behringer X-Air Series
Multitracker should be compatible with any Class Compliant audio interface. Just try it out.