How do I dynamically change the number of Niagara particles spawned after receiving a collision event?

Development Asset Creation FX
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1

Hello. In Niagara, I have a simple setup where an emitter generates a collision event, and another one receives it. I want to spawn particles from the receiving emitter when it catches the event, but I want the number of particles spawned be dependent on certain other things. This value can only be set to a fix number manually, or to random:

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Is there a way to do this?

2

I have some question as you , there even can’t set a User Parameter. I’m so confused.