clearInterval() doesn't clear interval in React

I would suggest you to return a clean up function so you don't register the interval twice in case you are in StrictMode with React 18, and also to remove it from the memory when the component gets unmounted.

Also use a ref sets with useRef and a separate useEffect that would watch changes in users and clear the interval there. Like so:

import { useEffect, useRef, useState } from "react";
const Cards = () => {
  const [users, setUsers] = useState(40);
  const intervalRef = useRef();

  useEffect(() => {
    if (users >= 5000) {
      console.log("ok");
      clearInterval(intervalRef.current);
    }
  }, [users]);

  useEffect(() => {
    intervalRef.current = setInterval(() => {
      setUsers((prevUsers) => (prevUsers = prevUsers + 100));
    }, 200);
    return () => clearInterval(intervalRef.current);
  }, []);

  return <div>number of users {users} </div>;
};

The way how you check for your condition users >= 5000 is not working because users is not listed as a dependency in your useEffect hook. Therefore the hook only runs once but doesnt run again when users change. Because of that you only check for 40 >= 5000 once at the beginning.

An easier way to handle that is without a setInterval way.

export const Cards = () => {
  const [users, setUsers] = useState(40);
  
  useEffect(() => {
    // your break condition
    if (users >= 5000) return;
    
    const increment = async () => {
      // your interval
      await new Promise((resolve) => setTimeout(resolve, 200));
      setUsers((prevState) => prevState + 100);
    }
    
    // call your callback
    increment();
    
    // make the function run when users change.
  }, [users]);
  
  return <p>current number of users {users}</p>
}

This doesnt work because:

• you never call the useEffect again to check if the condition is met
• the interval ref is lost

I made a working sample of your code here : https://codepen.io/aSH-uncover/pen/wvmYdNy

Addintionnaly you should clean the interval when the component is destroyed by returning the cleanInterval call in the hook that created the inteerval

const Card = ({ step }) => {
  const intervals = useRef({})
  
  const [users, setUsers] = useState(40)
  
  useEffect(() => {
    intervals.users = setInterval(() => {
      setUsers((prevUsers) => prevUsers = prevUsers + step)
    }, 200)
    return () => clearInterval(intervals.users)
  }, [])
  
  useEffect(() => {
    if (users >= 5000) {
        clearInterval(intervals.users)
    }
  }, [users])
  
  return (<div>number of users {users} </div>)
}

I came up with this. You can try it out. Although there are many ways suggested above

 const [users, setUsers] = useState(40);
 const [max_user, setMaxUser] = useState(true);

  let setIntervalUsers: any;
  let sprevUsers = 0;
  useEffect(() => {
    if (max_user) {
      setIntervalUsers = setInterval(() => {
        sprevUsers += 100;
        if (sprevUsers >= 5000) {
          setMaxUser(false);
          clearInterval(setIntervalUsers);
        } else {
          setUsers(sprevUsers);
        }
      }, 200);
    }
  }, []);