For recording directly from a guitar and this synth in a reasonably complete manner, you will want at least three inputs: one high-impedance instrument input for the guitar, and two line inputs for the synth. A common input configuration (once you have more than 2 inputs) is two inputs that can be mic (with phantom power) or high-impedance (possibly also line, and sometimes only one high-impedance) and two more line inputs.
However, you might want to pick off at least the guitar acoustically from your amp. That's where the mic input(s) come in handy.
Since your synth has stereo outputs, it is likely that you want to record both for most versatile results. If you think you can make do with just a single channel (and add stereo effects in the mix rather than using what the synth might provide on its own), you might get away with a 2-channel audio interface.
If you always record Midi in parallel, you'll still retain the option if you have a take worth keeping to replay the Midi to the synth and only then record the synth analog output in stereo. The original mono recorded track should give you a good reference for correcting the timing of that additional take so that you can then combine the mono guitar track with the stereo track produced from the original Midi.
Personally, I'd rather go for a 4-channel+Midi interface but in a pinch, you can muddle through with 2 channels in that manner.