There isn’t a set amount of money for you to earn before going self-employed. It depends how much you’re earning and how much money you need for your outgoings.
If you’re self-employed, you’re entitled to a tax-free personal allowance before you start getting charged tax. The standard personal allowance for the 2020/21 tax year is  £12,500.
If you have more than one job and one is self-employed, you still only get one personal allowance. Your personal allowance is applied to what HMRC sees as your main employment. Typically, your personal allowance is applied to the job paying the most.