If I understand it well, because I want to use a DC splitter (see image below) with a power supply 5V-10A, instead of using 2 diodes in the main cord, it is better to use number of devices x 2 diodes = 2 diodes on every split instead of 2 on main cord.
Yes - diodes after the split.
It's a bit complicated as this graph shows :
The voltage drop (FORWARD VOLTAGE) across each diode changes as the current (FORWARD CURRENT) goes up - but not linearly like it would with a resistor. For currents up to about 1 amp, the drop across each diode will be about 0.7 Volts.
So if you want to reduce 5V to 4.3V you need to use one diode. To reduce to 3.6V used two diodes in series. Three diodes will get you down to 2.9V.
So you want the diodes after each split so that the voltage drop stays closer to 0.7V, the current draw from the each camera does not affect the other cameras, and you don't put more total current through the diodes than they can handle.
Also, remember that diodes have polarity - you have to pay attention to which end of the diode goes towards the power supply and which end goes towards the camera. And when using more than one diode they have to be connected in series. If this is not clear let me know and I'll explain it in more detail.
