Bad idea to adopt cat and tell parents I randomly found it?

Ok so I moved back home after college and already have 2 cats that are 4. Couple months ago one of my cats had worms and the discussion was if needed to we could get another cat. I think they would be reluctant to me adopting another one just cause but will come around. So if I adopt a cat from the shelter and tell my mom I found it randomly at a plaza or something she’ll most likely be understanding. Idk if it’s a bad idea to do that or if I should do it

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