Start with the room, not the processor
A speaker layout should be drawn around actual seating positions, sightlines and available wall or ceiling locations. A 9.1.6 system only helps when the room has enough physical separation for wides and six height channels to form distinct directions.
What the numbers mean
The first number is the base-layer speaker count, the second is the number of subwoofer channels in the format description, and the last number is the height-layer count. In real cinemas there may be several physical subwoofers even when the soundtrack layout is written as .1.
5.1.4
A strong minimum immersive layout for smaller rooms: three screen channels, two surrounds and four height speakers. It can be excellent when seating is compact and there is no useful space for rear surrounds.
7.1.4
Adds rear surrounds, creating stronger front-to-back base-layer movement and better envelopment in rooms with seating away from the back wall.
9.1.6
Adds front wides plus an extra pair of height channels. This becomes valuable in longer rooms and larger seating plans, but only when the angular separation is meaningful.
More speakers can make calibration harder
Every additional speaker adds another coverage pattern, reflection field, delay, level and crossover relationship. Add channels when they improve spatial resolution - not simply because the processor has outputs.
Apply the principle to your room.
Send the room dimensions, seating plan and proposed system.