That's never an easy question. It depends on how you are casting it.
Are you slush it with resin as a limited run?
Are you casting solid and production runs?
From the pics, if you were slushing, you could go one piece, fill the deep recesses and place the gate on top. The tapered design allows for easy removal and you'd only have to worry about bubble entrapment (if at all) in the bridge of the nose and lower teeth.
If you're casting solid, and want to keep the deep recesses, I'd go 2 or 3 piece depending on where your seem will be.
Best case scenario with sculpting is have your mold plan before you even start. A mold is the most important part of the whole process since it dictates everything. It save a lot of time redoing details and texture if you know going in where everything will be in terms of gates, vents and seems.