I am at the beginning of a drone project, which will have a 25 kg MTOW, and I am analyzing whether I use four, six or eigth engines.
Is there a good advantage of using more motors or does it really depend on the project design?
Does anyone know a simulator that can help?
Adding more motors will generally give you more power, meaning you can lift more. You will however need a bigger battery to get the same hover time, since more motors will use more power as well. So there is a trade off between being able to lift a lot, and flying for a longer time.
Another advantage of more motors is that you can maintain maneuverability even if you lose a motor. So if you are carrying some expensive equipment or doing something critical, six or even eight motors might be preferable.
I'm not aware of any simulators that can test this accurately, but you can use this online calculator to test different configurations. I assume that is close to what you want? Most of the options require a payment, but you can probably get an idea of what you want without that.
Contrary to the other answer, a single larger propeller is more efficient than multiple smaller propellers. Adding motors (especially in a coaxial configuration) to achieve a required lift should be the last step, not the first, and should more be done for redundancy than anything else.
25KG is well within the lifting capacity of 4 reasonably sized motor/prop combinations (in the 20-24" range). Dropping this to 8 ~15" props will improve redundancy, but at the cost of efficiency (to some degree, see below).
The efficiency overhead of an electric motor is generally less than that of something like a gasoline engine which has very high parasitic losses (it takes a lot of energy for it just to stay running). The major overhead is weight of the motor, associated mounting hardware, and controller. For this reason, a machine built to be able to lift a payload with 4 motors, then upgraded to 8 motors, is a good idea as the additional motors will do more than lift their weight, at the monetary expense of having to run a bigger battery to do the same amount of work.
While you haven't commented on your previous piloting experience, this question seems rudimentary, and raises concern. A multirotor lifting 25kg is, quite literally, a flying blender and will severely injure or quite easily kill anyone that happens to be in its way. It's critical to heed some extreme caution and only allow experienced pilots (who can pilot without relying on superfluous sensors like GPS, magnetometer, and even accelerometer) fly.
