A simple way to assess how much will be "enough" is to multiply the required monthly income by 300. So, if you want £2000 a month as a pension/investment income, you would need to accumulate £600,000 in your pot at retirement.
This is based on taking a withdrawal of 4% per annum from your portfolio. Investing in the right asset classes will make this easier to achieve. Some people wish to leave a legacy, others not. This too will affect the amount of capital required.
Whilst this is a rule of thumb, nothing beats proper financial planning using cashflow software, which is by far the best way to get some idea on whether you are going to be "okay" or not.