You would have to edit the planet entity files themselves to designate the number of asteroids - there's no way to make it randomized though unless you add "clone" planet types with the varying resources and throw them into the random generator to use.

You can't, unfortunately. Like I said in my first reply, if you want to make randomish resources in 1.03, you'd have to clone the planet and then add it to the template for the game to use.

For example, asteroids have 1 ore and 1 crystal. You can make another asteroid with the exact same everything but instead have 2 ore and 2 crystal. Then you can open galaxyScenarioDef.galaxyScenarioDef, and look for which planet templates asteroids are listed in (should be something like Colonizeables and Asteroid), and expand it to include yours. Now random maps that use templates will have 2 to pick from, so some will be the original 1-1 rock, and some will be your 2-2 rock.

It's not the most elegant solution, but it works - hopefully random resources will make their way back on their own

To be fair, Gauntlet (another beta tester) came up with the method way back then, when resources weren't randomized either

Also note it may not work completely on the official maps - depends on whether how/what templates they use, but anything randomized should work.