You need X new users a month where X = 700 * churn rate. So if you churn 10% you need 70 new users a month to stabilise at 700. It's a hard goal BTW but might

You need X new users a month where X = 700 * churn rate. So if you churn 10% you need 70 new users a month to stabilise at 700. It's a hard goal BTW but might depend on how lucky you are. I.e. easy if you are lucky but hard for most.