I checked the mods, unfortunately the one that does allow more kids is really buged, and hasn't been updated since. If I recall correctly. 2018?It's just two kids unfortunately. Even if you divorce there's no more kids allowed with the new spouse.
Although there's mods to allow more.
Oh my God, thank you!The Polyamory Sweet mod - allows for mutiple spouses and to be able to customise the number of children you have (last updated 4 weeks ago). I use this mod and it works well.
There's a cool Immersive Family mod which allows your kids to be NPC's that interact with you, Jas and Vincent. It sounds good but I haven't downloaded it yet (updated last December). Just thought I'd mention it as it's kid related and seems fun.
Good luck :)