Hi, and welcome!
A couple of don'ts:
Don't spend money on a mixer - you don't need it, at least for the limited amount of what you have said you want to do.
Don't confuse the number of headphones you can drive with the number of separate headphone mixes you might need. If you are happy with having only one or two mixes but need to drive more physical headphones, then get a low-cost multi-channel headphone amplifier that you connect to the outputs on your interface.