24.
And you definitely need a fuse in each string. One false move and pffft!(A whole string).
With 24 in a string... I'd probably still keep a resistor in each one. Why? Because as your string has more and more LEDs in it the chances of the two having uneven total string voltages increases. That would result in in one string hogging more of the current.
I would do everything just as I directed at the top of this thread but I'd just use many more LEDs in each string owing to your higher supply voltage. Any other way may work but at much higher risk to your expensive LEDs, and greater likelyhood of one string being much brighter than the other.
If your entire LED count is south of 24 then one string could go with out the resistors. Keep the fuse.
There is another gotcha you need to be aware of. Your motor controllers could output pulsing DC not DC. Motors are inductive and will integrate(mathematically) the current pulses to result in some average voltage(90V). The motors wouldn't care if actually you were feeding them rectified 169V pulses. They would still believe they are getting 90VDC. If your supplies do not provide any filtering you could be hitting your stings with 150V pulses that would AVERAGE to 90VDC. Except the LEDs will not integrate, (average), the voltage!
I would NOT hook one of those supplies to an expensive set of LEDs before I knew exactly what was coming out of them. I would hook a 100W incandescent bulb to that supply's output and then CAREFULLY look at the waveform with an oscilloscope. (Note there are many ways to blow up the scope and/or kill yourself if you do that wrong.) There might be some alternatives to an O-Scope for figuring out what waveform is coming out of those supplies.
