It's simply a counter. 500 is just from the two RL examples SZ500 and SZ100 (the number is the max quantity). I just did a blanket example of 500 for a simplicity sake and it covers most maximum ammo loads for He 109/190. You could do 300, 200, 1000, whatever it doesn't matter.
You guys are getting hung up on this percentage thing. At 500 as in the example, the counters
would never be full but you can see the marks at 300rds for the MG 131, 200 or 150 for MG151. Again the slider just marks the count, it may be "full" or not depending on the maximum quantity setting and what is loaded.