Limit/Reduce number of Core in App Engine Flex Instance

I'm hosting my back end project on Google Cloud (App Engine Flex Instance), for now I have only 10 users but they charge me 250$ per month now, because I use several core, and so I used 2400 hours of accumulated instance time. Insane for only 10 users and not so much traffic! Can I reduce or limit the number of core used by my back end?