The thrust from each thruster depends on what it’s commanded to and its power supply. We provide a variety of performance charts you can refer to in the T200 technical details, which tell you how much thrust you can expect to get from a T200 given an ESC command and your supply voltage, as well as how much current is drawn to achieve that thrust.
The total horizontal thrust is composed of the thrust components from each of the horizontal thrusters, which depend on the commanded motion direction.
Yes, more thrusters generally results in more thrust, although it’s important to note the limitations of your power supply. If it can’t supply sufficient current for the thruster outputs you command then the control electronics will become underpowered and the entire ROV will restart (in which case you lose control, telemetry, and video, and can’t do anything other than wait for it to reboot and reconnect).
There are several ways to reduce the likelihood of losing power, but if you’re limiting thrust to avoid power outages then adding extra thrusters is unlikely to help things. You may want/need a higher capacity battery, or to run multiple batteries in parallel.