It’s very doable, but I wouldn’t call it “achieving immortality” in the literal sense. In his framework, “achieving immortality” really means creating and nurturing an energetic body. You can call it double, other, or whatever you want to call. There’s no hard limit, you could make a double, a triple, or ten more if you wanted.
Whether it needs to be fed depends on how it’s created. So you have to feed it for it to stay alive, or you don't need to feed it at all. Likewise, depending on how it’s created, it ceases to exist when you die, or it can live on independently.
This ties into your other topic about whether a practitioner’s magick can continue after they pass. Your physical body here on earth still dies either way. The “immortality” claim is really that the double can continue on after you’re gone, which is a different thing than you living forever in your current body.
