I think [MAX_SITE_POP_NUMBER:120] is actually the number of entities on a single site, the wiki mention "Generic site types will not hold any more than this number of entities", not the number of individuals on a single site (and it's probably it considering i have regularly noticed goblin site with more than 4000 goblins ... on a pocket world)
In the RAW you have [MAX_POP_NUMBER:10000] , meaning that 1 entity of the defined civ can be up to 10.000 individuals.
[MAX_STARTING_CIV_NUMBER:100] means that a civilization can have at max 100 entities and from the wiki the total number of a race is MAX_POP_NUMBER x MAX_STARTING_CIV_NUMBER , and with those default, we're talking about a potential of 1.000.000 of individuals !
Enough to cripple any computer if an adventurer move close to there.
But fortunately those numbers are reworked by the worldgen entry [TOTAL_CIV_POPULATION:15000] that for every world type has its default value to 15.000 individuals
Now this go along the
[SITE_CAP:...] (different value for different world size) that control the max amount of sites
[TOTAL_CIV_NUMBER:...] (different value for different world size) that control the max amount of civilization (not entities)
So you need to tailor carefully the numbers to avoid insane amount of individuals in a site and crippling the adventure mode by lagging extremely when moving near those sites.
Especially on smaller world that will have fewer sites and will get have to get a very lot of individuals in them then, due to those worlds still have the max pop of 15000 from worldgen and the max 10000 pop/entities from the raw, with the default values, you can easily get unplayable areas around dark fortresses