Translation:In this prison there are many criminals and few guards.
A better English translation might be, "In this prison, the criminals are many and the guards are few."
It's using sok and kevés as predicative adjectives, so they are the predicates, and you don't need a verb to be (part of) the predicate.
And a better Hungarian translation of "In this prison, there are many criminals and few guards" might be Ebben a börtönben sok bűnöző és kevés őr van (note the absence of a(z) in this case).
Many thanks Csabocza and many thanks guilth for posing the question because it's exactly what I was thinking. I have heard Hungarians speaking like this. With certain makes of cars, for example, sok a hiba or sok hiba van. As Mizinamo explains, the distinction is between the faults are many and there are many faults for which van would be required. I hope I have understood correctly.