Does North Korea's aircraft registration prefix allow for only 500 aircraft?
Civil aircraft tail numbers almost always follow ITU radio prefixes. North Korea has been allocated six ITU prefixes: HM and P5-P9.
Many countries with consecutive 2-letter prefixes will move the dash so it looks like they have a 1-letter prefix, but they still only use the allowed subset. So in this case, rather than P5-00 to P9-99, they write P-500 to P-999. They could have just as easily chosen P-50000 to P-99999 if they wanted, or gone alphabetic for even more possibilities.
However, in practice North Korea only has a handful of very old civil aircraft. While there is a humanitarian exception for spare parts to keep their airlines safe, the country can’t buy any more planes, which means they don’t need more tail numbers than they’re currently using. Heck, they could probably knock off a digit and still have more than they need!