Connecting two keyboards to Macbook via audio interface

Question

I have a question about connecting multiple keyboards to my Macbook for recording audio from the instruments themselves. I'm a newbie to these things so please bear with me.

Situation:

So currently I have a Yamaha CP73 connected to my Macbook via USB. The keyboard sends audio and MIDI signals out to the computer using just a USB cable. I have the onboard instrument sounds from CP73 playing through DAW for recording. Although it's not ideal, it works anyway, so I've been putting off the decision to get an audio interface.

Problem:

I'm now thinking of getting another smaller keyboard (Yamaha reface DX). I read that the reface DX doesn't have the ability to send out audio signals via USB, and needs an audio interface-to-Macbook setup to record its onboard sounds. So now I need an audio interface, but I never used one before, and I'm unsure if what I've understood so far from googling is right or wrong.

Questions:

I'm looking for a way to have these two keyboards connected to DAW for audio recording (not MIDI or VST). So the questions are:

1. Is multiple input audio interface the way to go?

2. If so, what kind of audio interface should I get? Like for e.g., I'm looking at Focusrite Scarlett 4i4 3rd Gen (random Google search result). It has two line inputs for two mic/instruments, but considering how a keyboard has L/R outputs, should I be looking for audio interfaces with four line inputs instead?

3. Is there a more cost-efficient way to do this other than getting an audio interface with four line inputs? Like, am I missing something here? Any advice is welcome.

Thanks in advance.

Answer 1

So you need an interface for the DX which you could also use for the CP73 or you could just add this in as an additional audio device. With Mac OSX you can create aggregate devices which means you can use both the 'interface' and the 'CP73' at the same time.

If you're on windows, you'll need a multichannel interface as it's more difficult to aggregate devices. You're right that you will need multiple line inputs to cover both keyboards. Also consider if you are going to use midi or any vocals - you'll need an additional preamp for a vocal mic.

No, you're not missing anything.