My goal is to increase the litter-size when pigs breed (to make them more desirable to farm), my first thought was to use EntityJoinWorldEvent conditioned on the entity being a baby pig and then spawning some random number more. However, it didn't take long to realize that spawning pigs every time a pig is spawned won't lead to

 

spawning pigs every time a pig is spawned won't lead to

spawning pigs every time a pig is spawned won't lead to

spawning pigs every time a pig is spawned won't lead to

spawning pigs every time a pig is spawned won't lead to

 

a result I desire... (mind my recursion joke). Any better way to do this? (Taking a look at the BabyEntitySpawnEvent, it seems I will run into the same issue).

BabyEntitySpawnEvent only fires on breeding, so it wouldn't cause the recursion you mentioned as long as you configure the additional babies manually.

Analyze the BreedGoal#spawnBaby method to see how you should configure the additional babies.

Also, don't forget to use a lower event priority and check if the event is canceled before doing anything in case another mod wants to cancel the spawning.