The number of people involved should be at least the same as those who worked on SMT IV. So the credits of that game should give a fair indication of the team size (not counting hired contractors).
I bet the SMT team is going through hell with this game because they haven't worked on a current-gen console game since the PS2 era. The SMT games went straight from PS2 (Nocturne) to DS (Strange Journey, Devil Survivor) and 3DS (IV/IV Apocalypse). Looks like they lack experience with making HD assets.