So every round a threat level is rolled, anywhere from under 10 to 100. This threat level is basically the "currency" that will be expended to create various antags, each antag costs a certain amount of currency.
For example,
Traitor costs 10, so if a total threat level of 40 was rolled roundstart, 4 traitors could potentially be spawned.
However there's also an antag cap variable which determines how many traitors can be spawned at various pop levels.
So at lower pops only one traitor could be spawned, while at higher pops up to 3 could be spawned.
Finally, theres the requirement variable, which sets a minimum requirement on the threat level per a set pop range, for traitor to be allowed to spawn.
These together, along with the pop range could look like this,
Pop range is static, while the threat level requirements and antag cap can be changed based on what seems appropriate balance wise, for example you wouldn't want multiple teams of nuke ops spawning, so you'd probably always keep their antag cap to 1.
(Thanks to Skoglol who was instrumental in teaching me how this stuff works.) I'll update this as I learn more information.