It depends on many factors.
First, let's say it is about 500 kcmil 1-2 kV pvc insulated and not jacketed.4 cables per set.
As per NEC Cptr. 9 Table 5 overall diameter 0.82 "-0.528 sqr.in
art.392.22 Number of Conductors or Cables.
(B) Number of Single-Conductor Cables, Rated 2000 Volts or less, in Cable Trays.
(1) Ladder or Ventilated Trough Cable Trays.
(b) Where all of the cables are from 250 kcmil through 900 kcmil, see Column 1 of Table 392.22(B)(1):
Cables total area 12*0.528=6.34 sqr.in.
As per Table 392.22(A) cable tray width 6" minimum.
However, according to 392.80 A (2) (b) ampacity it will the minimum:
Table 310.17 Ampacities of Single-Insulated Conductors in Free Air 65% only.
500 mcm copper :0.65*620[75oC] 0.65*700 A [90oC] alum. 0.65*485 and 0.65*545 A
403/455 A copper 315/354 A aluminum.
c) Where single conductors are installed in a single layer in uncovered cable trays, with a maintained space of not less than one cable diameter between individual conductors,
Table 310.17 Ampacities of Single-Insulated Conductors in Free Air [100%]
620/700 cu 485/545 al. [maximum]
Wdth=24*0.82=19.68"
Exception to (2)(c): For solid bottom cable trays, the ampacity of single
conductor cables shall be determined by 310.14(B). [You have to calculate the ampacity].
(d) Where single conductors are installed in a triangular or square configuration in uncovered cable trays, with a maintained free airspace of not less than 2.15 times one conductor
310.20 Ampacities of Conductors Supported on a Messenger:
500 mcm 75oC copper 496 alum. 392 A
Wdth=5*2.15*0.82=8.8” minimum
