I'd go for 5, that's 4 Companions + your PC, it's the same as your would find at most Tabletops (I've tried to DM for more and it tends to make any turn-based elements of the game a slog as people tend to lose interest as it takes so long for their turn to come around).
Larian have already indicated that there may be the option to add custom party members in a similar way to the IWD games and if they do then with 4 Companions you can have 2 of the Origin NPCs (for storyline purposes and animated cutscenes) and 2 custom ones to fill out the roster.