Hi,
I would say it depends from the type of music, how many LAB you need for an audience of 5000 open air.
Let's talk about numbers.
if you would use 4-5 stacks along the stage with 6-8 LABs on the Left and Right side and 2-3 times 4 pieces in the front of the stage, and then you "ARC" the horizontal sub array to get a virtualy point source, you can get a quite even sub distribution.
Driving one LAB with 2500-3000 watt capable amp(this would be then the peak power), you would get a max.
SPL@50m of 126dB and @100m 120dB...
Those numbers are based on my 10m measurements, and not verifyed at 50m and 100m!
However I always prefer by Rave and Techno music to increase the number of boxes and decrease the driving power to protect the drivers when tzhey should sound 10 hours long...
For normal rock, hip hop and rap or pop materials these numbers are even too mutch for 5000 people
I think the minimum quantity would be 3x or 4x 4 pieces of LABs using the "ARC" technique described above.
All the best,
Tako Tamas