Need guidance on how can I write a series of "rule based" code efficiently?

Hi guys,

I'm looking for resources on how can I write a series of "rule-based" code efficiently.

What I mean by rule-based is kinda like having a UI for user

IF quantity [remains] [3], [alert user]
IF quantity [less than] [2], [email admin]
etc etc..

I'm thinking how can I create this without having thousands of IF statements.

Any help would be greatly appreciated!