In this Unreal Engine 5 Niagara tutorial, we break down everything you need to know about the Spawn Rate module! Learn how spawn rate works per second, how to utilize the probability multiplier for randomizing particle counts, and how to effectively use Spawn Groups and masks to control multiple shapes and behaviors within a single emitter. 
https://dev.epicgames.com/community/learning/tutorials/9m9an/unreal-engine-ue5-niagara-basic-spawn-rate-10